Progress Report: Impact of coastal environment exposure on the

structural performance of recycled plastic planks

Author, Student ID: Stella Brown, 510560

Supervisor: Anna Wrobel-Tobiszewska

October 2022

Project Objectives

This project aims to recommend a recycled plastic lumber plank design out of two tested that

is most appropriate for use in coastal environments. Following exposure to different

environmental conditions, the recommended design will maintain appropriate design strength

and serviceability as well as minimal observed physical surface degradation. This project

contributes to research on the degradation of recycled plastic products exposed to coastal and

marine environments, as the number of plastic lumber decking installations has increased but

limited documentation exists of their long-term performance and degradation after exposure to

different environmental conditions. This research will improve the existing understanding of

the impact of different coastal environment conditions on the surface degradation and structural

strength of different recycled plastic plank compositions. Specific objectives of this project are:

i. Observing the surface degradation of recycled plastic planks when exposed to

environmental variables including ultraviolet radiation, seawater, and elevated

temperatures.

ii. Structural performance comparison between two recycled plastic plank designs

exposed to such variables.

iii. Recommendation of the recycled plastic plank design most suited for use in coastal

environments.

Summary of Work Completed

The work completed to date will be detailed in the following paragraphs with reference to

relevant sections of the Draft Thesis attached. The project objectives are shown above for

context and have been altered slightly after completing market research and a thorough

literature review where now only two different recycled plastic planks designs are being

considered. The literature review has significantly informed and developed the proposed

methods and project approach.

The initial portion of the project focused on selecting recycled plastic plank designs for testing.

Research was conducted to determine plank design possibilities, where it was originally

proposed that planks could be manufactured by 3D printing to better control variables. Despite

this, manufacturing planks was not a viable option, and it was decided to use planks available

on the market. Various suppliers of recycled plastic decking were contacted and Ekodeck and

Replas were willing to provide samples free of charge. Background on the development,

composition and properties of recycled plastic decking planks, particularly the types available

on the Australian market, is discussed in Section 2.2.1. This provides context on the designs

that will be used for testing. More detail is also provided on the selected designs in Sections

3.1.1 and 3.1.2.

Work that has been ongoing for most of the semester has been the development of methods to

expose the plank designs to coastal environment conditions. Different variables were explored

including freshwater, seawater, wave activity, temperature, and UV radiation. After

investigating the availability of testing facilities within different Schools at the University of

Tasmania, it was finally decided to focus on seawater, UV radiation, and temperature. Facilities

were explored within the Australian Maritime College, School of Natural Sciences, School of

Chemistry, Tasmanian Institute of Agriculture, IMAS and the School of Engineering. Focusing

on three different variables also narrowed the scope of the project as to not spread resources

too thin. Alongside surveying available facilities, literature was reviewed to determine how

coastal exposure has previously been simulated to promote weathering of plastic lumber

materials. From reviewing literature, it was decided that to promote degradation in the UV

radiation and temperature treatments, samples should be periodically doused with water, as

shown in Sections 2.4.1 and 2.4.2 which review literature on weathering of plastic lumber

materials due to UV radiation and temperature exposure. The methods currently proposed for

the environmental exposure treatments are detailed in Section 3.2. A review of literature

detailing weathering of plastic lumber materials due to seawater exposure is detailed in Section

2.4.3, and a review of literature examining the performance of recycled plastic decking and

similar products in coastal environments is detailed in Section 2.3.2.

Significant work has also gone into determining how degradation of the plank designs will be

measured after exposure to the environmental treatments. From the Project Plan, it was decided

that two different degradation measurements would be used, including structural testing and a

method to determine degradation or changes to mass or surface profile. After performing a

review of literature, a decision was made to use ASTM International Standard D6109 to

determine changes to structural properties, specifically flexural properties. More standards

exist that specify how to test different properties of recycled plastic decking planks, but

following more than one standard would increase the number of samples required and hence

resources required for environmental exposure. Flexural properties were also commonly

examined when determining changes to structural properties of plastic lumber materials

following weathering. A review of literature focusing on methods for assessing the structural

properties of lumber is shown in Section 2.2.2. Methods for structural testing are detailed in

Section 3.3.1. Different methods were explored for measuring degradation of the plank samples,

including FTIR spectroscopy, mass changes, and water filtering, but a decision was made to

use electron microscopy to examine physical surface degradation. While not presenting

quantifiable results, it can still show changes to surface profiles which can provide an indication

of how degradation occurs for different plank designs under different exposure conditions. A

review of literature examining the degradation of plastic lumber materials using electron

microscopy is detailed in Section 2.4.5. Methods for measuring physical degradation using

electron microscopy are detailed in Section 3.3.2.

Literature and background information on the uptake of recycled plastic products, including

decking, and motivations for the adoption of recycled plastic decking and related products in

outdoor environments has been reviewed and discussed in Sections 2.1 and 2.3.1.

Guide to Draft Thesis

1. Introduction

This section provides a brief introduction and context to the project, including motivations and

objectives. This section is near completion and will be finalised before submission of the final

thesis. A brief background is provided on reasons for the increased popularity of recycled

plastic decking products, and degradation of recycled plastic decking in coastal environments.

The research approach is introduced, including an overview of methods for environmental

exposure and structural and degradation testing. The project objectives, and assumptions and

limitations of the project, are also described.

2. Literature Review

This section is near completion and will be added to as required. This section introduces and

reviews literature on the development and application of recycled plastic decking in coastal

environments, the performance and degradation of recycled plastic decking and related

products in coastal environments, and different methods to simulate coastal environment

exposure and measure the degradation of recycled plastic products. It is anticipated that review

of additional literature on environmental exposure methods and degradation testing will be

required.

3. Methods

This section is near completion and has been informed by the Literature Review. This section

details the properties of the recycled plastic decking samples used for testing, and methods to

prepare the samples prior to environmental exposure, expose the samples to different

environmental conditions, test structural properties, and observe physical degradation.

Additional details are required for different testing equipment used, and timings of

environmental exposure.

4. Results

Results are yet to be obtained so this section is incomplete. It is expected this section will

display results of structural testing and electron microscopy observations for exposed and

unexposed samples of each plank design.

5. Discussion

This section will be completed after final results have been collected. The results will be

discussed to provide a recommendation of the plank design best suited for use in coastal

environments, assess the suitability of recycled plastic planks used in coastal environments,

sources of error and limitations of the project, and recommendations for further work.

6. Conclusion

The conclusion will be completed after the discussion of results and final recommendations

have been made. This section will present the final findings and recommendations presented

in the discussion, including a summary of the observed surface degradation and changes to

structural properties of the plank designs after environmental exposure, and the final

recommended design.

Highlighted comments are shown in the draft thesis. Comments in GREEN at the start of

sections serve to provide context for the section, and future additions to be made to the section.

Comments in YELLOW indicate tasks yet to be completed.

Plan for Completion

A Gantt chart is shown in Appendix A detailing the tasks and timeline for project completion.

After submission of the draft thesis, final methods and resources will be finalised to begin

exposure of plank samples to environmental treatments at the start of November 2022. Further

confirmation is required on the availability of ultraviolet radiation and temperature exposure

equipment, which will contribute to the timings of environmental testing. Further reference of

literature will also be required to finalise exposure methods. Required risk assessments will be

prepared and submitted for approval to the School of Engineering and IMAS for environmental

exposure testing once final details are confirmed. Exposure of plank samples to environmental

treatments will be conducted between November 2022 and March 2023, with structural testing

of exposed plank samples and control plank samples to occur in March 2023. Surface

degradation observations for the exposed and unexposed plank samples will be performed

using electron microscopy in March 2023. Following testing and observations, data analysis,

discussions and recommendations will be completed in April/May 2023 to deliver the final

project objectives.

Comments on Progress

The progress made on this project has been largely within the timeframes specified in the

Project Plan. Not all final details have been organised to begin testing at the time of the Progress

Report submission as was anticipated, although this was an ambitious target. Some of the

milestones specified in the plan have been altered, where it is now anticipated that structural

testing of all plank samples will occur in April 2023, rather than separate testing in October

2022 and April 2023. It is still expected that exposure of plank samples to environmental

conditions will be completed by April 2023, and other milestones as specified in the Project

Plan will be adhered to or exceeded.

Availability of most resources required to complete the project have been arranged, but there

are some remaining project risks due to reliance on third parties and availability of equipment.

Planks from Ekodeck have been delivered, with planks from Replas expected for delivery in

October 2022. The availability of resources in the Engineering Workshop to cut planks to size

in preparation for environmental exposure treatments in October 2022 has been confirmed.

Preliminary arrangements have been made to organise equipment required to perform structural

tests in the Engineering Workshop in March 2023 and will be finalised closer to this date. The

availability of equipment to condition samples at the Tasmanian Institute of Agriculture prior

to structural testing in March 2023 has been confirmed and will be finalised close to this date.

The availability of electron microscopy facilities at the School of Chemistry to perform surface

degradation testing in March 2023 has been confirmed and will be finalised closer to this date.

Arrangements for seawater environmental exposure at IMAS Taroona have been finalised with

the required facilities booked from end of October 2022 to end of March 2023 and ready for

use. Equipment required for temperature and UV radiation environmental exposure at IMAS

Salamanca are available, but final arrangements are still being made in regard to scheduling

and determining exact exposure periods. In the event these facilities are unavailable, an oven

in the Engineering Workshop is available, and alternative UV radiation equipment has

availability at the School of Natural Sciences. An option exists to design and construct a custom

UV exposure chamber, but this would require significant additional resources that have not

been factored into the scope of this project.

Appendix A: Gantt Chart

Made using TeamGantt.

Impact of coastal environment exposure on the

structural performance of recycled plastic planks

S. R. Brown, 510560

October 2022

School of Engineering

This thesis is submitted in partial fulfilment of the requirements for the

degree of Bachelor of Engineering with Honours, University of Tasmania

Declaration

This Thesis to the best of my knowledge and belief contains no material published

or unpublished that was written by another person, nor any material that infringes

copyright, nor any material that has been accepted for a degree or diploma by

University of Tasmania or any other institution, except by way of background in-

formation and where due acknowledgement is made in the text of the Thesis.

This Thesis is the result of my own investigations, except where otherwise stated.

Other sources are acknowledged in the text giving explicit references. A list of

references is appended.

I hereby give consent for my Thesis to be available for photocopying, inter-

library loan, electronic access to University of Tasmania staff and students via the

University of Tasmania library, and for the title and summary to be made available

to outside organisations, in accordance with the Copyright Act 1968.

Signed:

Dated

i

ii

Abstract

To be completed before final thesis submission

iii

iv

Acknowledgements

v

vi

Contents

Declaration ........................................................................................................................ i

Abstract ............................................................................................................................ iii

Acknowledgements ........................................................................................................... v

Nomenclature ................................................................................................................ xiii

1 Introduction .............................................................................................................. 1

1.1 Background ................................................................................................................ 1

1.2 Research objectives .................................................................................................... 2

1.3 Research approach .................................................................................................... 2

1.4 Assumptions and limitations ..................................................................................... 3

2 Literature Review ..................................................................................................... 5

2.1 Recycled plastic .......................................................................................................... 5

2.2 Development and prevalence of recycled plastic decking ...................................... 6

2.2.1 Background and development of plastic lumber .................................................................. 6

2.2.2 Assessing the structural performance of plastic lumber ....................................................... 8

2.3 Recycled plastic decking in coastal environments .................................................. 9

2.3.1 Adoption of plastic lumber in the environment ................................................................... 9

2.3.2 Performance and degradation of plastic lumber structures in coastal environments .......... 10

2.4 Simulation of coastal environmental variables for weathering plastic lumber

materials ................................................................................................................................ 12

2.4.1 Ultraviolet exposure ........................................................................................................... 12

2.4.2 Temperature ....................................................................................................................... 14

2.4.3 Seawater exposure ............................................................................................................. 14

2.4.4 Summary ............................................................................................................................ 15

2.4.5 Measuring plastic lumber degradation ............................................................................... 16

3 Experimental Method ............................................................................................ 19

3.1 Sample preparation ................................................................................................. 19

3.1.1 Plank design selection ........................................................................................................ 19

3.1.2 Plank design properties and manufacture........................................................................... 20

3.1.3 Plank sample preparation ................................................................................................... 20

vii

3.2 Environmental treatments ...................................................................................... 21

3.2.1 Ultraviolet exposure ........................................................................................................... 21

3.2.2 Temperature exposure ........................................................................................................ 21

3.2.3 Seawater exposure .............................................................................................................. 22

3.3 Performance assessment .......................................................................................... 22

3.3.1 Structural performance ....................................................................................................... 22

3.3.2 Physical degradation .......................................................................................................... 25

4 Results ..................................................................................................................... 27

4.1 Structural performance ........................................................................................... 27

4.1.1 Maximum fibre stress ......................................................................................................... 27

4.1.2 Flexural strength (Modulus of rupture) .............................................................................. 27

4.1.3 Maximum strain ................................................................................................................. 27

4.1.4 Modulus of elasticity .......................................................................................................... 27

4.1.5 Secant modulus of elasticity ............................................................................................... 27

4.2 Physical degradation ................................................................................................ 27

5 Discussion ............................................................................................................... 29

5.1 Recommended product for use in coastal environments ...................................... 29

5.2 Suitability of plastic lumber decking for use in coastal environments ................ 29

5.3 Sources of error and limitations ............................................................................. 29

5.4 Future investigations ............................................................................................... 29

6 Conclusion .............................................................................................................. 31

References ....................................................................................................................... 33

Appendices ...................................................................................................................... 39

Appendix A: ........................................................................................................................... 39

viii

List of Figures

Figure 1: Micrographs captured from electron microscopy showing surface degradation

of HDPE/wood-flour composite samples (a) before accelerated weathering, and after

accelerated weathering for (b) 1000 hours, (c) 2000 hours, and (d) 3000 hours (Stark et

al., 2004). ........................................................................................................................ 16

Figure 2: Micrographs captured from electron microscopy showing surface degradation

of LDPE before (a) and after (b, c) accelerated weathering under different conditions

(Gulmine et al., 2003). .................................................................................................... 17

Figure 3: UV radiation exposure chamber used for testing at IMAS Salamanca. .......... 21

Figure 4: Ovens used for temperature exposure at IMAS Salamanca. ........................... 22

Figure 5: Tanks used for seawater exposure at IMAS Taroona. .................................... 22

Figure 6: Samples prepared for exposure to environmental treatments. ........................ 23

Figure 7: MTS 810 Material Testing System used for structural testing. ....................... 23

Figure 8: SEM facilities at the School of Chemistry, UTAS. ......................................... 25

ix

x

List of Tables

Table 1: Plastic lumber products used for testing. HDPE stands for high-density

polyethylene and LDPE stands for low-density polyethylene. ....................................... 19

xi

xii

Nomenclature

List of Abbreviations and Acronyms

FTIR Fourier Transform Infrared Spectroscopy

HDPE High-density Polyethylene

IMAS Institute for Marine and Antarctic Studies

LDPE Low-density Polyethylene

SEM Electron Microscopy

TIA Tasmanian Institute of Agriculture

UTAS University of Tasmania

UV Ultraviolet

WPC Wood-plastic Composite

Symbols and Units

% Percent

C Celsius

AU$ Australian dollar

nm Nanometre

mm Millimetre

cm Centimetre

m Metre

xiii

xiv

List of Appendices

Appendix A: .................................................................................................................... 39

xv

xvi

1 Introduction

Mostly complete, details may be added to for the research approach, and assumptions and

limitations. Will be updated before final submission.

1.1 Background

Since plastics started being mass produced in the 1950s, over 6 billion tonnes of plastic

waste have been generated, and more than 11 million tonnes are discharged into oceans

annually (UN Environment Programme, 2022, Rhodes, 2018, MacLeod et al., 2021). In

response to this growing environmental issue, major organisations and governing bodies

have proposed initiatives to reduce the volumes of plastic waste entering environments

focused on increasing the volume of plastic being recycled or reprocessed (van der Vegt

et al., 2022, Commonwealth of Australia, 2018, Department of Agriculture, 2021). This

recognition has increased the popularity and availability of recycled plastic products in

global and Australian markets, including new installations in coastal environments

utilising decking planks composed of recycled plastics and other materials such as wood

flour (Locock et al., 2017, Riddle, 2014).

The increased uptake of such decking planks reduces the volume of plastic entering

marine environments and contributes to the development of a plastic circular economy,

but exposure to degrading conditions present in coastal environments such as seawater,

high temperatures, and solar radiation can negatively impact the structural properties,

lifespan, and recyclability of plastic products (Breslin et al., 1998, Lopez et al., 2006,

Iñiguez et al., 2018). As one example, a 1998 study observed that the bending modulus

of recycled plastic decking in a pier structure exposed to the marine environment varied

by 50% over a two-year period (Breslin et al.). Degradation of plastic products in coastal

environments can also contribute to marine pollution from the generation of microplastics,

but there is limited research available on how recycled plastic products may contribute to

this environmental issue (Enfrin et al., 2022, Andrady et al., 2022, Da Costa et al., 2018).

This study analyses the degradation and structural performance of two different plastic

lumber decking products with different compositions available on the Australian market

when exposed to coastal environmental variables. Gaining a better understanding of the

performance and degradation of recycled plastic decking planks under different

1

environmental conditions would improve product design and could also reduce plastic

pollution from its use.

1.2 Research objectives

This project aims to recommend a recycled plastic plank design out of two tested that is

most appropriate for use in coastal environments. Following exposure to different

environmental conditions, the recommended design will maintain appropriate design

strength and serviceability as well as minimal observed physical surface degradation.

This project contributes to research on the degradation of recycled plastic products

exposed to coastal and marine environments, as the number of plastic lumber decking

installations has increased but limited documentation exists of their long-term

performance and degradation after exposure to different environmental conditions. This

research will improve the existing understanding of the impact of different coastal

environment conditions on the surface degradation and structural strength of different

recycled plastic plank compositions. Specific objectives of this project are:

iv. Observing the surface degradation of recycled plastic planks when exposed to

environmental variables including ultraviolet radiation, seawater, and elevated

temperatures.

v. Structural performance comparison between two recycled plastic plank designs

exposed to such variables.

vi. Recommendation of the recycled plastic plank design most suited for use in

coastal environments.

1.3 Research approach

Two recycled plastic decking plank products were selected for testing of their degradation

and structural performance before and after environmental exposure after performing a

literature review and conducting market research. The first design was composed of

approximately 100% recycled plastic and the second plastic design was composed of

approximately 50% recycled plastic and 50% reclaimed timber fill.

To simulate environmental conditions, including prolonged exposure to flowing seawater,

ultraviolet radiation and elevated temperatures, water tanks, ovens, and ultraviolet

radiation equipment available at the Institute for Marine and Antarctic Studies (IMAS)

2

were used. The specific conditions for environmental exposure such as exposure times,

apparatus, temperatures, and water conditions were determined through a literature

review and the limitations of equipment and other resources available for the project. The

structural performance of the plank designs was tested before and after environmental

exposure using ASTM International standard D6109 to measure flexural properties.

Surface degradation of the planks was assessed by observing changes using electron

microscopy.

Final recommendations were based on a comparative analysis between the different plank

designs tested, assessing different properties such as product cost and changes to

structural performance and surface degradation following environmental exposure.

1.4 Assumptions and limitations

The primary limitation of this research is the availability of equipment to simulate

different environmental conditions. The decking plank samples will be limited to 65 cm

in length due to the dimensions of the equipment used to simulate different temperature

conditions, compared to a larger recommended sample length dictated by ASTM

International standard D6109. The time of environmental exposure is also limited by time

available to complete the project and scheduled availability of equipment. The

degradation of samples exposed to ultraviolent radiation may not be completely reflective

of realistic environmental exposure conditions due to equipment with the desired

radiation wavelength not being available.

3

4

2 Literature Review

Most required background and literature has been covered in this review. More

information will be added to sections as needed, including methods and impacts of

environmental exposure on plastic lumber materials.

2.1 Recycled plastic

Annually 380 million tonnes of plastic are produced with less than 20% of this total being

recycled, and more than 8 million tonnes entering the world’s oceans (Zhao et al., 2022).

In Australia only 13% is recycled, with 84% of plastic waste being sent to landfill

(Department of Agriculture, 2021). The Australian Department of Agriculture estimates

that AU$419 million is lost in economic value each year from plastics not being recycled

and instead being incinerated, or disposed in landfill and oceans where it can take up to

500 years to completely degrade (Department of Agriculture, 2021, Zhao et al., 2022).

Plastic pollution from discarded plastic presents significant social, economic, and

environmental consequences, particularly as microplastics, a result of plastic degradation

from environmental exposure, have become recognised as an issue of increasing concern

(Department of Agriculture, 2021, UN Environment Programme, 2022). Microplastics

are plastic debris less than 5 mm in length, and are easily absorbed or ingested by plants

and animals which can lead to accumulation in food chains (Rhodes, 2018, Zhao et al.,

2022). For example, earthworms that ingest microplastics in soils can be a source of

microplastics in chicken, which can have serious health implications if ingested by

humans (Zhao et al., 2022). It is already estimated that a single person ingests up to

113,000 plastic particles annually (Department of Agriculture, 2021). Considering this

issue, movements have been made in recent years to create a circular economy for plastics,

with plastic recycling being encouraged by governing organisations to reduce pollution

and its consequences (Commonwealth of Australia, 2018, Department of Agriculture,

2021). In Australia, the case for investing in plastic recycling initiatives has increased as

several Asian countries have begun imposing recycling waste import restrictions, where

Asia was previously a primary destination for unwanted or unvaluable recycled waste

(Cáceres Ruiz and Zaman, 2022).

One of the most common plastic types recycled is high-density polyethylene (HDPE),

making up 33% of plastic recycled globally (Locock et al., 2017). Significant growth is

5

projected for the use of recycled HDPE, being a more cost-effective material for

manufacturing products when compared to virgin plastic (Locock et al., 2017). Recycled

HDPE is the primary polymer type used in recycled plastic decking due to its favourable

and repeatable engineering properties (Rodrigues, 2022, Locock et al., 2017). Demand

for recycled plastic decking in Australia is expected to grow alongside the projection for

increased rates of HDPE recycling, presenting an opportunity for recycled plastic decking

manufacturers and suppliers (Locock et al., 2017, Rodrigues, 2022, Vaughan Levitzke,

2019). Despite this opportunity for the development and use of recycled plastic products

such as decking planks, there is limited research on their degradation and life cycle

environmental impacts. Recycled plastic products may have similar potential

environmental impacts as virgin plastic products regarding microplastic generation and

landfill disposal for example. A recent study by Enfrin et al. (2022) examined

microplastic generation from pavements constructed from asphalt mix that included

recycled plastic waste, where methods were proposed to determine microplastic

generation from such pavements to help inform organisations that may choose to use such

a product in assessing potential environmental impacts prior to construction. A similar

approach could be applied to installations in coastal environments that use recycled

plastic products such as decking as there is limited research into their long-term

performance and environmental impacts in such environments.

2.2 Development and prevalence of recycled plastic decking

2.2.1 Background and development of plastic lumber

Recycled plastic decking planks are a type of plastic lumber. Plastic lumber is defined as

a manufactured lumber product composed of more than 50% resin, generally of a

rectangular cross-section (ASTM International, 2003). The plastic lumber industry

originated in the 1970s in Japan and Europe from the use of thermoplastic products as a

more available alternative to wood (Lampo and Nosker, 1997, Nosker and Renfree, 2000).

Post-industrial plastic waste was used as it was low-cost and most available, but had

weakness in regard to a reduced flexural modulus when compared to wood and being less

suitable for structural applications, but was recognised as being more environmentally

sustainable and not as readily degradable (Nosker and Renfree, 2000). In the United

States in the mid-1980s, low-density polyethylene (LDPE) and high-density polyethylene

6

(HDPE) from commingled recycled plastic was identified as being a more suitable

material for plastic lumber as it had more repeatable materials, produced a compressive

strength comparable to that of wood, and adequate stiffness (Lampo and Nosker, 1997,

Nosker and Renfree, 2000). This application for LDPE and HDPE provided an

opportunity for recycling centres where these plastics were viewed as a liability, or

curbside tailings with negative value and destined for landfill (Lampo and Nosker, 1997).

In the present-day HDPE has been adopted as the primary polymer used in plastic lumber

polymer matrices due to producing better properties for structural applications (Rodrigues,

2022, Locock et al., 2017).

Another composition of plastic lumber that has been developed to improve the product’s

engineering properties and resistance to degradation is wood-plastic composite (WPC)

lumber. WPC is a type of plastic lumber that was first manufactured by Mobil Chemical

in 1996, where wood fibres are dispersed in the polymer matrix to improve the

engineering properties of plastic lumber, including greater strength and stiffness, and less

creep and expansion (Wilson, 1999, Rodrigues, 2022). This type of lumber is

representative of the majority of plastic lumber available in the Australian market. Other

compositions of plastic lumber have been developed to contain varying proportions of

fibreglass and additives, such as UV stabilisers, to improve the product’s engineering

properties and resistance to degradation in outdoor environments and for use in structural

applications (Nguyen et al., 2018, Bowders et al., 2003, Breslin et al., 1998).

The most popular plastic lumber decking in Australia is WPC decking, with HDPE being

the primary plastic used, combined most often with wood flour, a by-product of the timber

industry (Riddle, 2014). Two of the most prominent brands of plastic lumber in Australia

are Replas and Ekodeck. Replas is an Australian manufacturer of plastic lumber decking

without wood composite, while Ekodeck is an Australian distributer of WPC, readily

available at major hardware stores such as Bunnings (Bunnings, Replas). Other

Australian distributers include Integrated Recycling, Enduradeck, and APR Composites,

which all sell WPC decking with varying proportions of recycled plastic, timber, and

additives. Overall, there is limited information available on the history and applications

of plastic lumber products in Australia, and their long-term performance in Australian

conditions.

7

2.2.2 Assessing the structural performance of plastic lumber

Several methods have been used to measure the structural performance of plastic lumber

since it began being considered for use in structural and other load-bearing applications.

Lampo and Nosker (1997) recognised that methods and standards that existed at the time

of publication for assessing the properties of plastic products were inadequate for

assessing the properties of plastic lumber due to their special composition and structure,

which typically has many voids and a widely varied composition between products.

Knights (1996) reported that the inadequacy of adequate standards also inhibited the use

of plastic lumber in structural applications, with there being no consistent performance

data and industry standards, which also inhibited market growth. In their research, Lampo

and Nosker (1997) contributed to the development of new ASTM International standards

that were introduced in 1997 and have since been widely adopted and recognised by

plastic lumber manufacturers. The purpose of the standards was to make product

performance more consistent and give more information on the suitability of plastic

lumber for structural applications (Knights, 1997, Knights, 1996). These standards make

the structural performance of plastic lumber more readily measured and standardised and

increases the confidence of end users in the adequacy of plastic lumber used in structural

applications (Wilson, 1999, Knights, 1997, Lampo and Nosker, 1997). The performance

standards include, but are not limited to, ASTM standards D6108, D6109, D6111, and

D6112 which dictate methods for determining flexural properties, compressive properties,

bulk density, specific gravity, and creep (Lampo and Nosker, 1997, Knights, 1997). These

performance standards can be used to directly compare the engineering performance of

different makes of plastic lumber.

Williams et al. (2015) used ASTM International standards D6108 and D6109 to assess

the suitability of plastic lumber for use as a timber substitute in mining applications.

Using the procedures set out in these standards, it was found that samples containing

approximately 65% LDPE and HDPE, 20% polystyrene, and 15% woodchip filler had

flexural and compressive properties that were similar to timbers used for certain

applications within the mining industry. Similarly, Bowders et al. (2003) used these

standards to determine whether plastic lumber pins made using LDPE and HDPE were

suitable for use in slope stabilisation.

8

Other studies that assess the structural performance of plastic lumber decking and its

suitability as a substitute for timber use other ASTM International standards, though this

is due to the tests being performed before the publication of specific plastic lumber testing

standards. For example, Carroll et al. (2003) and Breslin et al. (1998) both use ASTM

International standard D695 to determine the compressive strength and modulus of plastic

lumber used in structural applications, but use different methods and standards to

determine flexural properties meaning that their results may not be directly comparable.

Broadly following the requirements of ASTM International standards D6108, D6109, and

other ASTM International standards that have been developed specifically for assessing

the properties of plastic lumber within the limitations of the resources available for this

project will produce more consistent results for assessing the structural properties of

recycled plastic decking planks.

2.3 Recycled plastic decking in coastal environments

2.3.1 Adoption of plastic lumber in the environment

As the properties of plastic lumber have improved it has become more readily adopted in

outdoor and structural environments. The first use of plastic lumber was in low stress

outdoor applications where it could be easily substituted for wood, including park

benches and picnic tables (Nosker and Renfree, 2000). This has extended to more high-

stress structural applications such as bridges and pilings as developments in the

composition and reinforcement of plastic lumber has improved, particularly in the reuse

of post-consumer and post-industrial plastic waste (Nosker and Renfree, 2000,

Bajracharya et al., 2014). Aside from the development of recycled plastic lumber for

structural applications, plastic lumber has also been substituted for wood in environments

where wooden structures easily corrode and require constant maintenance or replacement.

For example, Lampo and Nosker (1997) reported on a water conservation and recreational

site in Lake Shelbyville, USA, where two wooden structures that required replacement

biannually due to degradation were substituted with plastic lumber. These two structures

were a floating goose nest that is continuously in water, and an observation shelter that

becomes partially submerged in the event of heavy rains and high water. Despite current

developments in the use of plastic lumber for structural applications, its primary uses are

still in low-stress environments such as decking, fencing and outdoor furniture, but where

9

it is no less exposed to degrading environmental factors (Locock et al., 2017, Wilson,

1999). Australian plastic lumber manufacturer Replas also reports several examples of

their products being used to replace wooden structures, as a more durable and low-

maintenance alternative (Replas, 2020a, Replas, 2020c, Replas, 2020b).

Another reason for plastic lumber decking being so widely adopted is its marketing as a

sustainable or low-cost alternative to traditional wood lumber. These factors motivated

the construction of plastic lumber vehicular bridges in conjunction with the U.S. Army

(Chandra and Kim, 2012). An “environmentally friendly” thermoplastic made from

almost 100% recycled postconsumer waste was used to construct bridges in Fort Bragg,

USA, and Fort Eustis, USA, capable of supporting 71 tonnes military tanks. The study by

Chandra and Kim (2012) claimed the bridges were environmentally sustainable due to

reducing the amount of plastic entering landfills, reducing forest degradation, and not

being susceptible to corrosion or rot. The uptake of recycled plastic products like plastic

lumber is also endorsed in Australia by the Federal Government as a sustainable practice

as part of the National Plastic Plan 2021, which encourages the uptake of recycled plastic

by government and business, also supported by the National Waste Policy 2018 where

one of the targets is to “significantly increase the use of recycled content”

(Commonwealth of Australia, 2018, Department of Agriculture, 2021). While recycled

plastic decking can reduce levels of deforestation and contribute to building a circular

economy of plastics to reduce plastic pollution, more extensive research needs to be

performed on the life-cycle environmental impacts of plastic lumber to consider more

than its original manufacture and implementation. This includes its recyclability after

long-term exposure to degrading environments such as coastal environments and impacts

plastic lumber may have on coastal environments in which it is installed (Iñiguez et al.,

2018). This would contribute to a more informed decision being made by organisations

and individuals who may decide to install plastic lumber decking in coastal environments

if sustainability or long-term serviceability are key motivations.

2.3.2 Performance and degradation of plastic lumber structures in coastal

environments

Coastal environments are harsh environments with more severe degrading properties,

particularly in Australia. This includes the presence of seawater, wave activity, sand

abrasion, higher wind speeds, solar radiation, and elevated temperatures. Coastal

10

environments are also home to diverse and sensitive ecosystems that can be easily

disrupted by contaminants, such as potential byproducts of plastic lumber degradation

(Rhodes, 2018). As previously discussed, part of the appeal of plastic lumber decking is

its resistance to weathering or degradation in harsh environments. Another factor

supporting the sustainability of plastic lumber decking is that it does not leach harmful

level of chemicals into the environment when compared to some treated timber products.

Construction of the Tiffany Street Pier in New York City, USA, was completed in 1995

to replace a decayed wooden pier, with the new pier consisting almost entirely of

postconsumer recycled LDPE and HDPE, with 4-6% other materials (Xie et al., 1997).

Plastic lumber was used as the primary construction material to reduce the rate of

degradation compared to traditional lumber. In an environmental assessment conducted

by Xie et al. (1997) on the impact of pier on surrounding water quality, it was concluded

that the recycled plastic lumber would not noticeably contribute to the river pollutant load

and had significant environmental advantages. This is supported by Lalonde et al. (2011)

who concluded that plastic wood samples used in their study of the toxicity of aquatic

construction materials were generally non-toxic when exposed to aquatic environments.

In another pier constructed in New York, USA, Breslin et al. (1998) studied the long-

term engineering properties of plastic lumber used for the decking. Over a two-year

exposure period where the decking was exposed to seasonal temperature and UV intensity

changes and water submersion, no noticeable degradation such as warping, discolouration

or cracking was observed, although a 50% variation in the modulus of elasticity was noted

over the period of the study. While the serviceability of the decking was maintained,

further investigation was recommended into long-term testing of plastic lumber used in

exposed structures due to the variation in engineering properties and the lack of research

to meet the service life claims of manufacturers. This lack of research is also observed by

Bowyer et al. (2010) in their review of wood-plastic composite (WPC) lumber, where it

is noted that the properties and lifespan of plastic lumber decking are defined by the

claims of manufacturers rather than long-term research into their degradation in exposed

environments. In fact, their review also notes that plastic lumber decking can encounter

the same issues as traditional wood decking with documented instances of

biodeterioration, UV degradation and discolouration. Plastic lumber may be a suitable

substitute for timber in low-stress applications where timber may be more susceptible to

corrosion, but more research is required into its degradation over extended periods as part

11

of existing structures to unequivocally support this assessment, or concern over how this

degradation could also release solid waste such as microplastics, not just chemical

leaching, and impact coastal and marine environments.

2.4 Simulation of coastal environmental variables for weathering

plastic lumber materials

Studies on the degradation of plastic composites show that humidity, ultraviolet radiation,

and temperature variation are key agents in the weathering process (Lopez et al., 2006).

In this study it is proposed that the structural performance and degradation of recycled

plastic decking be considered by exposing three sets of plank samples to different

treatments or variables. This review will focus on three key degrading elements of coastal

environmental conditions, including prolonged ultraviolet exposure or solar radiation,

high temperatures, and seawater.

2.4.1 Ultraviolet exposure

Ultraviolet (UV) radiation can cause irreversible changes to the chemical structures of

polymers, contributing to degradation, although most plastic lumber products limit this

degradation through the addition of UV stabilisers in their composition (Nguyen et al.,

2018, Lopez et al., 2006). UV radiation can be categorised into three wavelength bands:

UV-A, UV-B, and UV-C (McKenzie and Madronich, 2003). UV-A radiation has a

wavelength of 315-450 nm and constitutes almost all UV radiation reaching the Earth’s

surface and is the most penetrating (McKenzie and Madronich, 2003). UV-B radiation

has a wavelength of 280-315 nm but is mostly absorbed by the atmospheric o-zone layer

(McKenzie and Madronich, 2003). UV-C radiation has a wavelength of 100-290 nm and

reaches the Earth’s surface in negligible quantities (McKenzie and Madronich, 2003).

Ultraviolet radiation contributes to the degradation of plastic and wood-plastic

composites in the environment, with the UV-C wavelength spectrum being the most

effective at accelerating the weathering of polyethylene and contributing to physical

degradation due to higher energy, making it more efficient breaking polymer bonds

(Zvekic et al., 2022, Douminge et al., 2013, Martínez-Romo et al., 2015). A study by

Zvekic et al. (2022) found that LDPE weathered 10 times faster under a UV-C weathering

treatment than a UV-B weathering treatment.

12

Studies on the degradation and weathering of polyethylene and WPC due to UV exposure

typically use the UV-A and UV-B wavelength bands during artificial weathering

procedures as this is most reflective of outdoor conditions and how the materials would

naturally weather (Lopez et al., 2006, Friedrich and Luible, 2016). As an example, Lopez

et al. (2006) used UV wavelengths of 365 nm and 313 nm within the UV-A and UV-B

spectrums to more closely simulate natural daylight in their study on how outdoor

conditions impact the performance of natural-fibre-plastic composites. Stark et al. (2004)

employs a similar approach, using a wavelength range of 300-400 nm. Studies on the UV

weathering characteristics of WPCs and HDPE tend to be done using specialised

weathering apparatus, where UV exposure, temperature, humidity and simultaneous

water spray at set intervals is controlled (Stark et al., 2004, Friedrich and Luible, 2016,

Lopez et al., 2006, Falk et al., 2000). Water spray is often used during artificial

weathering trials as moisture in combination with solar, or ultraviolet, radiation is a key

contributor to polymer and WPC weathering (Lopez et al., 2006). Moisture contributes

to physical degradation by imposing physical forces on sample cracks and pores when it

is absorbed or desorbed (Lopez et al., 2006). This observation is supported by Stark

(2005), where it was observed that the degradation of HDPE/wood-flour composites

under UV exposure was limited in dry conditions compared to when moisture was present

along with UV exposure (Stark, 2006). Machado et al. (2016) also states that the

engineering properties of WPCs, including bending strength and modulus of elasticity,

are more negatively affected by exposure to a high humidity than by temperature or UV

exposure, where wetting and drying cycles generate internal stresses and can also

encourage biodegradation in wood composites. Stark (2005) noted that water spray in

combination with UV light encouraged more surface degradation than water spray or UV

exposure on its own, as the water spray would wash away the surface layer degraded from

UV exposure and the remaining surface would be compromised and more prone to

degradation due to dimensional surface changes.

Considering this, a set of plank samples could be exposed to UV radiation along with a

source of moisture to accelerate degradation over the testing period. This would assess

the impact of UV exposure on the degradation of plastic lumber decking. The samples

could be exposed to a UV source of a known wavelength, with samples being periodically

removed from the source to introduce moisture and increase degradation by dousing the

samples in distilled water.

13

2.4.2 Temperature

Temperature is an important variable to consider in the weathering of plastic lumber

materials, as it causes expansions and contractions in polymers and WPCs, where

combined with the presence of moisture fatigue can occur in polymer bonds, contributing

to physical degradation (Halabe et al., 2007). Elevated temperatures are associated with

reduced flexural properties, as observed by Dutta et al. (2006) and Carroll et al. (2003).

Polymers tend to hold heat for longer, as noted by Ekodeck and Lampo and Nosker (1997),

where plastic lumber decking reaches higher temperatures than timber decking and

retains the heat for longer after outdoor exposure on sunny days. Ekodeck has observed

that on a clear 29 C day that their composite decking products reached a surface

temperature of approximately 65 to 70 C, which was comparable to some hardwoods

tested, while a study by Halabe et al. (2007) observed a fibre reinforced composite bridge

deck reached a temperature of 50 C under similar conditions, although the composite

was covered in a 9.5 mm wearing surface consisting of an aggregate blend which may

have reduced the observed temperature.

Similar to the environmental treatment proposed for UV exposure conditions, changes in

moisture during an environmental treatment involving high temperatures would

encourage greater degradation over the available exposure period for this study, as well

as periods where the polymer matrix can expand and contract due to temperature

fluctuations. To simulate prolonged exposure to high temperatures, a set of planks should

be exposed to a temperature that their surface temperature would be expected during a

hot summer day, or within the range of 50 C and 70 C, to simulate the environmental

conditions. To allow temperature variations and introduce moisture, plastic lumber

samples could be periodically allowed to cool to room temperature and generously

doused in distilled water.

2.4.3 Seawater exposure

Exposure to seawater can degrade the flexural properties of polymers and WPCs to a

greater extent than regular distilled water (Najafi and Kordkheili, 2011). Recently this

environmental factor has become of concern to researchers as the issue of microplastics,

a potential product of recycled plastic plank weathering, has become more prevalent.

Seawater has properties present that can encourage the degradation of polymers and wood

14

compared to fresh water, including biological matter, salinity, and acidity (Rutkowska et

al., 2002, Najafi and Kordkheili, 2011). A study by Najafi and Kordkheili (2011)

immersed WPCs in distilled water, and seawater, and observed that the samples immersed

in seawater had a greater reduction in flexural properties. This reduction in flexural

properties increased as the proportion of wood flour in the composite samples increased.

HDPE is prone to physical degradation and generating microplastics in seawater

compared to other polymers, particularly in combination with the presence of UV

radiation (Naik et al., 2020, Da Costa et al., 2018). Despite this, immersion in seawater

may not accelerate degradation as much as variations in moisture, as shown by Andrady

(1990). Andrady (1990) found that thermoplastic materials exposed outdoors in seawater

tended to degrade at a slower rate than materials exposed outdoors in the open air.

Polymer photodegradation in marine environments from UV exposure primarily occurs

on beaches and near to the water surface, due to seawater preventing the build-up of heat

or inhibiting UV radiation penetration (Andrady, 1990, Andrady et al., 2022). More

research is required to understand how plastic lumber materials, particularly those

available in the current market, degrade in the presence of seawater and in combination

with other variables considering the susceptibility of HDPE to generate microplastics. As

previously discussed, this would contribute to understanding how recycled plastic

decking impacts the environments in which it is used.

As part of this study, it is proposed that a set of plastic lumber plank samples be exposed

to flowing seawater to test the degradation level as a response to this environmental

condition. While degradation may be limited without exposure to other variables such as

UV radiation and larger temperature variations, it could demonstrate differences in the

suitability of performance of different plastic lumber compositions, where seawater has

been shown to have more degrading qualities as the proportion of organic matter in plastic

lumber increases.

2.4.4 Summary

Overall, coastal conditions expose plastic lumber decking planks to a wide variety of

degrading conditions that can impact dimensional stability, engineering performance,

discolouration, and produce microplastics. Coastal environments expose plastic lumber

elements to ultraviolent or solar radiation, biological attack, and moisture. While seawater

has been shown to be have more impact on degradation to plastic lumber materials than

15

distilled water, there are still limited studies on the durability and degradation of plastic

lumber when exposed to coastal conditions, particularly seawater.

2.4.5 Measuring plastic lumber degradation

Aside from changes to engineering properties, methods such as Fourier transform infrared

spectroscopy (FTIR) and electron microscopy (SEM) can be used to observe the

degradation of plastic lumber. FTIR is generally used to observe chemical changes in

surface structures of samples after weathering, while SEM can be used to observe

physical changes. Physical changes are of most interest to this study; physical degradation

observed by SEM could indicate the potential for the generation of microplastics and

environmental impacts of samples from use in coastal environments. Several studies use

SEM to examine the surface degradation of polymers and wood-polymer composites after

natural and artificial accelerated weathering, with these facilities available for use at the

University of Tasmania (Stark et al., 2004, Rasouli et al., 2016, Taib et al., 2010, Gulmine

et al., 2003, Ozdemir and Mengeloglu, 2008). Micrographs captured using SEM

displaying surface degradation of plastic lumber materials are shown in Figure 1 and

Figure 2.

Figure 1: Micrographs captured from electron microscopy showing surface degradation

of HDPE/wood-flour composite samples (a) before accelerated weathering, and after

accelerated weathering for (b) 1000 hours, (c) 2000 hours, and (d) 3000 hours (Stark et

al., 2004).

16

Figure 2: Micrographs captured from electron microscopy showing surface degradation

of LDPE before (a) and after (b, c) accelerated weathering under different conditions

(Gulmine et al., 2003).

Degradation due to weathering, including UV radiation and water spray, can cause visible

surface cracking and voids that can be observed using SEM, as shown by studies by Stark

et al. (2004) and Gulmine et al. (2003), with micrographs demonstrating these

observations in Figure 1 and Figure 2 respectively. Taib et al. (2010) used SEM to observe

the surface degradation of different HDPE WPCs exposed to natural weathering, noting

that microcracks occurred in the polymer matrix of HDPE during weathering, and that

wood fibres present in WPCs if not properly encapsulated by the polymer matrix became

become protruded and flaked off, creating voids. Using SEM in this study could

determine whether physical changes occur to the surface of samples from exposure to

different environmental treatments. This would indicate the susceptibility of the different

designs to degradation in different conditions, and the potential propensity to generate

microplastics.

17

18

3 Experimental Method

Almost complete, some further specifics are required regarding equipment and

environmental variable exposure times.

3.1 Sample preparation

3.1.1 Plank design selection

Two manufactured recycled plastic decking plank products available on the Australian

market were used for experimental testing. The designs used were Replas Enduroplank™,

and Ekodeck® Decking Classic. These two designs have compositions with varying

proportions of recycled post-consumer plastic and are marketed as a sustainable

alternative to traditional timber decking (Replas, 2019, Ekodeck). The Replas product is

a plastic lumber product, consisting of 98% post-consumer and post-industrial plastic,

and 2% UV stabiliser and colourants (Replas, 2019). The Ekodeck product is a WPC with

48% recycled HDPE, 45% reclaimed timber, and 7% additives including UV stabilisers,

coupling, colourants, anti-mould agents (Ekodeck). The specifications for each lumber

profile used are summarised in Table 1.

Table 1: Plastic lumber products used for testing. HDPE stands for high-density

polyethylene and LDPE stands for low-density polyethylene.

Product Composition Dimensions Source

Replas 98% HDPE and LDPE, 2% UV 195 x 55 mm (Replas,

Enduroplank™ stabiliser and colour 2019)

Ekodeck® Decking 48% recycled HDPE, 45% 137 x 23 mm (Ekodeck)

Classic reclaimed timber, 7% additives

Initially, it was proposed that planks could be manufacturer using 3D printing. This way,

the properties of the planks would be more controlled. Of key interest was controlling the

proportion of recycled plastic in the planks, to determine whether this impacted the extent

of degradation and changes to structural properties following coastal environment

exposure. This option was deemed to be unfeasible, as there was no way to control the

recycled plastic content in 3-D printing filament within the resource constraints of this

project. As a result, the decision to test on products available on the Australian market

19

was made. Using products available on the market would also allow a better

understanding to be developed on the suitability of recycled plastic planks currently being

used in Australian coastal environments.

3.1.2 Plank design properties and manufacture

Replas Enduroplank is a plastic lumber decking plank consisting of approximately 98%

post-consumer and post-industrial mixed plastic and 2% additives. From information

available from Replas, the product is marketed as a maintenance free, ethical alternative

to timber, with an expected lifespan of over 40 years (Replas, 2019). No toxic substances

are reported to be generated from its use, but colour fade due to UV exposure is expected

(Replas). There are several examples of Replas products, including Enduroplank, being

used in coastal and other harsh environments as a replacement for timber structures that

had previously corroded, as well as other installations that are reported to still being in

excellent condition after more than 15 years in service (Replas, 2020a, Replas, 2020c,

Replas, 2020b). Replas Enduroplank is typically used for commercial purposes, including

decks, footpaths, bridges and piers.

Ekodeck Decking Classic is a wood-plastic composite plastic lumber decking product

consisting of 45% reclaimed timber, 48% recycled HDPE, and 7% additives including

UV stabilisers, coupling, colourants, and anti-mould agents (Ekodeck). The product

comes with a 10-year warranty and is reported to be rot, decay, and termite resistant

(Ekodeck). Ekodeck Decking Classic is typically used in residential and commercial

purposes for outdoor decks.

More detail required on method of plank product manufacture and source of materials in

composition

3.1.3 Plank sample preparation

20 samples, each 600 mm in length, were cut for each of the plank designs. ASTM D6109

dictates that the support span for plastic lumber samples undergoing flexural testing

should be 16 (with a tolerance of +4, -2) times the depth of the plastic lumber profile,

with the length of the samples being a minimum of 10% greater than this support span

length to allow for additional overhang (ASTM International, 2003). For the Ekodeck

samples this would mean a minimum sample length of 354 mm, and for the Replas

samples 847 mm. However, the size of the samples is limited to the size of the smallest

20

environmental testing apparatus with the internal dimensions of the ovens only being 650

mm x 650 mm, and the UV enclosure having dimensions of 700 mm x 300 mm. This

requires the length of the Replas samples to be less than required by the ASTM D6109

standard, or approximately 600 mm. For a length of 600 mm, 29.2% less than prescribed

by the standard. While this will not inhibit calculation of the percentage change in flexural

properties after the environmental treatments, the flexural properties calculated will be

inaccurate.

All the preparation of the samples, including measurements and cutting was completed

in the School of Engineering workshop, University of Tasmania, October 2022.

3.2 Environmental treatments

Exact timings of ultraviolet radiation exposure, as well as for other treatments, will be

further confirmed and specified on basis of literature and equipment availability.

Five samples of each plank type were exposed to test procedures that replicate different

variables for coastal environmental conditions. This includes ultraviolet radiation,

flowing seawater, and high temperatures. Five samples were kept as a control. Samples

were exposed to one exposure treatment for a period of approximately 10 weeks, or 1,600

hours, with degradation and changes to flexural properties due to this exposure being

measured.

3.2.1 Ultraviolet exposure

Five samples were exposed to a UV exposure treatment using facilities available at the

Institute for Marine and Antarctic Studies in Hobart, Tasmania. Samples were exposed

to UV-C radiation at 254 nm in an ultraviolet chamber of dimensions 700 x 300 x 300

mm (length x width x depth). Samples were removed from the chamber twice a week and

thoroughly doused in distilled water.

Photo of UV chamber apparatus

Figure 3: UV radiation exposure chamber used for testing at IMAS Salamanca.

3.2.2 Temperature exposure

Five samples were exposed to a temperature treatment using facilities available at the

Institute for Marine and Antarctic Studies in Hobart, Tasmania. Samples were placed in

21

an oven at a temperature of 65 C, with internal dimensions of 650 x 650 mm. Samples

were allowed to cool overnight twice a week, then thoroughly doused in distilled water,

and returned to the oven at 65 C.

Photo of oven

Figure 4: Ovens used for temperature exposure at IMAS Salamanca.

3.2.3 Seawater exposure

Five samples were exposed to a treatment involving flowing seawater drawn directly

from the Derwent Estuary at the Institute for Marine and Antarctic Studies in Taroona,

Tasmania. Planks were placed in tanks where water flowed at a rate of 60 L/min, with

blackout blankets placed directly over the tanks to prevent UV exposure.

Photo of seawater tanks

Figure 5: Tanks used for seawater exposure at IMAS Taroona.

3.3 Performance assessment

3.3.1 Structural performance

At this stage it is anticipated that the MTS 810 Material Testing System will be used from

the Engineering Workshop, but other options are available and will be confirmed closer

to the testing date.

Changes to the structural properties of the planks in response to the environmental

treatments were measured against ASTM International standard D6109: “Standard Test

Methods for Flexural Properties of Unreinforced and Reinforced Plastic Lumber”, a four-

point bending test to failure or rupture for plastic lumber with rectangular cross-sections

(ASTM International, 2003). This standard is used to determine the flexural properties of

plastic lumber decking boards, including maximum fibre stress, flexural strength, flexural

yield strength, maximum strain, and modulus of elasticity. The flexural properties of the

15 samples exposed to the environmental treatments and the 5 control samples were tested

during a 24-hour period using the MTS 810 Material Testing System machine in the

School of Engineering Workshop, University of Tasmania. Prior to this testing being

performed, samples were prepared in facilities at the Tasmanian Institute of Agriculture.

Samples were conditioned at 23 C (+/- 2), 50% humidity (+/- 5) for 40 hours. When

22

performing the tests, the following procedure was used for each sample (ASTM

International, 2003):

1. The width of the sample at various points was measured along the sample’s

length to a precision of 1% and the average value recorded. This process was

repeated for the depth of the sample.

2. The required support span was determined, and the support span was set to be

within 1% of this value. The support span was 16 (+4, -2) times the depth of the

sample, or as close to this value as possible.

3. The required rate of crosshead motion was calculated and for a load span one-

third of the support span, the machine was set to within 50% of this rate. The

rate of crosshead motion was calculated according to Equation 1.

Equation 1: Rate of crosshead motion from ASTM D6109.

R = 0.185ZL2/d

where:

R = rate of crosshead motion, mm/min

L = support span, mm

d = depth of the beam, mm

Z = rate of straining of the outer fibres, mm/mm/min. Z shall be equal to 0.01.

4. The loading noses and supports for the sample were aligned so that they were

parallel, and the load span was one-third the support span. The sample was

centred, with the long-axis perpendicular to supports.

5. Load was applied to the specimen at the required crosshead rate while

simultaneous load-deflection data was collected. Deflection was measured at the

centre of the span.

6. The test was continued until a break occurred in the sample.

Photo of samples (labelled with dimensions)

Figure 6: Samples prepared for exposure to environmental treatments.

Photo of testing machine

Figure 7: MTS 810 Material Testing System used for structural testing.

23

3.3.1.1 Maximum fibre stress

The maximum fibre stress was calculated for the outer fibres of the plastic lumber samples.

The stress was calculated using the deflection values on the load-deflection curve. The

following equation was used:

Equation 2: Maximum outer fibre stress equation from ASTM D6109.

S = PL/bd2

where:

S = stress in outer fibre throughout load span, MPa

P = load at a given point on the load-deflection curve, N

L = support span, mm

b = width of the beam, mm

d = depth of the beam, mm

3.3.1.2 Flexural strength

The flexural strength, or modulus of rupture, was determined from Equation 2 by letting

P be the load at which failure occurred.

3.3.1.3 Maximum strain

The maximum strain in the outer fibres of the plank samples were calculated from the

deflection data. The following equation was used:

Equation 3: Strain equation from ASTM D6109.

r = 4.70Dd/L2

where:

r = strain, mm/mm/min

D = midspan deflection, mm

L = support span, mm

d = depth of the beam, mm

3.3.1.4 Modulus of elasticity

The procedure to determine the modulus of elasticity of samples as specified by ASTM

D6109 will be written here.

3.3.1.5 Secant modulus of elasticity

The procedure to determine the secant modulus of elasticity of samples as specified by

ASTM D6109 will be written here.

24

3.3.2 Physical degradation

Specific details of the SEM equipment used, and specific procedures and conditions of

testing will be added closer to the testing date when final arrangements are made.

Electron microscopy facilities available at the School of Chemistry, University of

Tasmania, were used to examine physical changes to the surfaces of samples due to

degradation from the environmental treatments. One sample of each plank type from each

environmental treatment, and one sample of each plank type from the control samples, or

8 samples in total, were examined. The surface profile of the samples exposed to

environmental treatments were compared to the surface profile of the unexposed control

samples. Features that were compared include the smoothness of sample surfaces, and

the frequency and size of any present cracks, voids and splintering, similar to the observed

surface degradation in the micrographs in Figure 1 and Figure 2.

Periodically dousing samples exposed to the temperature and UV radiation environmental

treatments in water encouraged the physical degradation of samples. For samples exposed

to the temperature treatment, this further fatigued the polymer bonds during expansion

and contraction from the absorption and desorption of water particles. For samples

exposed to the UV radiation treatment, water removed degraded surface particles

exposing the remaining surface to further degradation alongside the added stresses of

absorption and desorption.

Photo of SEM facilities

Figure 8: SEM facilities at the School of Chemistry, UTAS.

25

26

4 Results

4.1 Structural performance

Results from structural testing of each set of samples. Including information on measured

dimensions, and measured ambient and outdoor conditions over the time period during

which environmental exposure occurred.

4.1.1 Maximum fibre stress

4.1.2 Flexural strength (Modulus of rupture)

4.1.3 Maximum strain

4.1.4 Modulus of elasticity

4.1.5 Secant modulus of elasticity

4.2 Physical degradation

Results of SEM analysis, including scaled images from testing.

27

28

5 Discussion

5.1 Recommended product for use in coastal environments

Discussion on performance of different products under different conditions, including

factors that would have contributed to their performance such as composition, dimensions,

and method of manufacture. Based on this performance, a recommendation on the

product best suited to use in coastal environments will be made with justification.

5.2 Suitability of plastic lumber decking for use in coastal

environments

Based on previous recommendations and literature review, a discussion on the suitability

of current plastic lumber decking available on the market for use in coastal environments,

including serviceability and environmental impacts.

5.3 Sources of error and limitations

Sources of error and limitation of study will be discussed, including apparatus etc. How

this may have impacted results, and how the experimental design could be improved.

5.4 Future investigations

Recommendations for future investigations to further assess suitability of recycled plastic

products used in natural environments.

29

30

6 Conclusion

Conclusion to be written after results have been collected and final recommendations

made.

31

32

References

ANDRADY, A. L. Environmental degradation of plastics under land and marine

exposure conditions. 1990.

ANDRADY, A. L., LAVENDER LAW, K., DONOHUE, J. & KOONGOLLA, B. 2022.

Accelerated degradation of low-density polyethylene in air and in sea water.

Science of The Total Environment, 811, 151368.

ASTM INTERNATIONAL 2003. ASTM D6108-03: Standard Test Method for

Compressive Properties of Plastic Lumber and Shapes. West Conshohocken:

ASTM International.

BAJRACHARYA, R. M., MANALO, A., KARUNASENA, W. & LAU, K.-T. A. Glass

fibre and recycled mixed plastic wastes: recent developments and applications.

In: SMITH, S. T., ed. 23rd Australasian Conference on the Mechanics of

Structures and Materials (ACMSM23), 2014 Byron Bay, NSW. Lismore. NSW:

Southern Cross University, 288-292.

BOWDERS, J. J., LOEHR, J. E., SALIM, H. & CHEN, C.-W. 2003. Engineering

Properties of Recycled Plastic Pins for Use in Slope Stabilization. Transportation

Research Record: Journal of the Transportation Research Board, 1849, 39-46.

BOWYER, J., FERNHOLZ, K., HOWE, J. & BRATKOVICH, S. 2010. Wood-Plastic

Composite Lumber vs. Wood Decking: A Comparison of Performance

Characteristics and Environmental Attributes.

BRESLIN, V. T., SENTURK, U. & BERNDT, C. C. 1998. Long-term engineering

properties of recycled plastic lumber used in pier construction. Resources,

Conservation and Recycling, 23, 243-258.

BUNNINGS. Ekodeck [Online]. Bunnings. Available:

https://www.bunnings.com.au/brands/e/ekodeck [Accessed 2022].

CÁCERES RUIZ, A. M. & ZAMAN, A. 2022. The Current State, Challenges, and

Opportunities of Recycling Plastics in Western Australia. Recycling, 7, 64.

CARROLL, D. R., STONE, R. B., SIRIGNANO, A. M., SAINDON, R. M., GOSE, S. C.

& FRIEDMAN, M. A. 2003. Structural properties of recycled hdpeplastic lumber

decking planks. International Journal of Polymeric Materials and Polymeric

Biomaterials, 52, 709-724.

33

CHANDRA, V. & KIM, J. S. World's First Recycled Plastic Bridges. International

Conference on Sustainable Design and Construction (ICSDC) 2011, 2012 Kansas

City, Missouri. 585-593.

COMMONWEALTH OF AUSTRALIA 2018. 2018 National Waste Policy. Canberra:

Commonwealth of Australia.

DA COSTA, J. P., NUNES, A. R., SANTOS, P. S. M., GIRÃO, A. V., DUARTE, A. C.

& ROCHA-SANTOS, T. 2018. Degradation of polyethylene microplastics in

seawater: Insights into the environmental degradation of polymers. Journal of

Environmental Science and Health, Part A, 53, 866-875.

DEPARTMENT OF AGRICULTURE, WATER AND THE ENVIRONMENT 2021.

National Plastics Plan 2021. Canberra.

DOUMINGE, L., FEAUGAS, X., BERNARD, J. & MALLARINO, S. 2013. Extrinsic

fluorescence as a sensitive method for studying photo-degradation of high density

polyethylene correlated with mechanical stresses. Current Applied Physics, 13,

1751-1757.

DUTTA, P. K., LOPEZ-ANIDO, R. & KWON, S.-C. 2006. Fatigue durability of FRP

composite bridge decks at extreme temperatures. International Journal of

Materials and Product Technology, 28, 198-216.

EKODECK. Classic / Technical Specifications [Online]. Ekodeck. Available:

https://www.ekodeck.com.au/products/composite-decking/classic/technical-

specifications/ [Accessed 2022].

EKODECK. Composite decking’s Eco-credentials [Online]. Ekodeck. Available:

https://www.ekodeck.com.au/composite-deckings-eco-credentials/ [Accessed

2022].

EKODECK. How Hot Does Composite Decking Get? [Online]. Ekodeck. Available:

https://www.ekodeck.com.au/how-hot-does-composite-decking-get/ [Accessed

2022].

ENFRIN, M., MYSZKA, R. & GIUSTOZZI, F. 2022. Paving roads with recycled

plastics: Microplastic pollution or eco-friendly solution? Journal of Hazardous

Materials, 437.

FALK, R., LUNDIN, T. & FELTON, C. The Effects of Weathering on Wood-

Thermoplastic Composites Intended for Outdoor Applications. Proceedings of

34

the 2nd Annual Conference on Durability and Disaster Mitigation in Wood-Frame

Housing, 2000 Madison, Wisconsin.

FRIEDRICH, D. & LUIBLE, A. 2016. Investigations on ageing of wood-plastic

composites for outdoor applications: a meta-analysis using empiric data derived

from diverse weathering trials. Construction and Building Materials, 124, 1142-

1152.

GULMINE, J. V., JANISSEK, P. R., HEISE, H. M. & AKCELRUD, L. 2003.

Degradation profile of polyethylene after artificial accelerated weathering.

Polymer Degradation and Stability, 79, 385-397.

HALABE, U. B., VASUDEVAN, A., KLINKHACHORN, P. & GANGARAO, H. V. S.

2007. Detection of subsurface defects in fiber reinforced polymer composite

bridge decks using digital infrared thermography. Nondestructive Testing and

Evaluation, 22, 155-175.

IÑIGUEZ, M. E., CONESA, J. A. & FULLANA, A. 2018. Recyclability of four types of

plastics exposed to UV irradiation in a marine environment. Waste Management,

79, 339-345.

KNIGHTS, M. 1996. Plastic lumber: ready for prime time. Plastics Technology 42.

KNIGHTS, M. 1997. Plastic lumber makers move to standardize testing. Plastics

Technology 43, 41-42.

LALONDE, B. A., ERNST, W., JULIEN, G., JACKMAN, P., DOE, K. & SCHAEFER,

R. 2011. A Comparative Toxicity Assessment of Materials Used in Aquatic

Construction. Archives of Environmental Contamination and Toxicology, 61,

368-375.

LAMPO, R. & NOSKER, T. J. 1997. Development and testing of plastic lumber materials

for construction applications.

LOCOCK, K., DEANE, J., KOSIOR, E., PRABAHARAN, H., SKIDMORE, M. &

HUTT, O. 2017. The recycled plastics market: Global analysis and trends.

Australia: CSIRO.

LOPEZ, J. L., SAIN, M. & COOPER, P. 2006. Performance of natural-fiber-plastic

composites under stress for outdoor applications: Effect of moisture, temperature,

and ultraviolet light exposure. Journal of Applied Polymer Science, 99, 2570-2577.

MACHADO, J. S., SANTOS, S., PINHO, F. F. S., LUÍS, F., ALVES, A., SIMÕES, R.

& RODRIGUES, J. C. 2016. Impact of high moisture conditions on the

35

serviceability performance of wood plastic composite decks. Materials & Design,

103, 122-131.

MACLEOD, M., ARP, H. P. H., TEKMAN, M. B. & JAHNKE, A. 2021. The global

threat from plastic pollution. Science, 373, 61-65.

MARTÍNEZ-ROMO, A., GONZÁLEZ-MOTA, R., SOTO-BERNAL, J. J. &

ROSALES-CANDELAS, I. 2015. Investigating the Degradability of HDPE,

LDPE, PE-BIO, and PE-OXO Films under UV-B Radiation. Journal of

Spectroscopy, 2015, 586514.

MCKENZIE, R. & MADRONICH, S. 2003. ULTRAVIOLET, SURFACE. In:

HOLTON, J. R. (ed.) Encyclopedia of Atmospheric Sciences. Academic Press.

NAIK, R. A., ROWLES, L. S., HOSSAIN, A. I., YEN, M., ALDOSSARY, R. M., APUL,

O. G., CONKLE, J. & SALEH, N. B. 2020. Microplastic particle versus fiber

generation during photo-transformation in simulated seawater. Science of The

Total Environment, 736, 139690.

NAJAFI, S. K. & KORDKHEILI, H. Y. 2011. Effect of sea water on water absorption

and flexural properties of wood-polypropylene composites. European Journal of

Wood and Wood Products, 69, 553-556.

NGUYEN, V. D., HAO, J. & WANG, W. 2018. Ultraviolet Weathering Performance of

High-Density Polyethylene/Wood-Flour Composites with a Basalt-Fiber-

Included Shell. Polymers, 10.

NOSKER, T. J. & RENFREE, R. Recycled plastic lumber: from park benches to bridges.

Approved for Proceedings of R’2000 5th World Congress, Toronto, Canada, 2000.

OZDEMIR, T. & MENGELOGLU, F. 2008. Some Properties of Composite Panels Made

from Wood Flour and Recycled Polyethylene. International Journal of Molecular

Sciences, 9, 2559-2569.

RASOULI, D., DINTCHEVA, N. T., FAEZIPOUR, M., LA MANTIA, F. P., MASTRI

FARAHANI, M. R. & TAJVIDI, M. 2016. Effect of nano zinc oxide as UV

stabilizer on the weathering performance of wood-polyethylene composite.

Polymer Degradation and Stability, 133, 85-91.

REPLAS. Replas Product Properties [Online]. Replas. [Accessed 2022].

REPLAS. 2019. Enduroplank™ [Online]. Replas. Available:

https://www.replas.com.au/products/decking/enduroplank/ [Accessed 2022].

36

REPLAS. 2020a. Enduroplank™ Decking Installed Along The Lake Foreshore at Lake

Buffey Griffin in Canberra, ACT [Online]. Replas. Available:

https://www.replas.com.au/enduroplank-decking-installed-along-the-lake-

foreshore-at-lake-buffey-griffin-in-canberra-act/ [Accessed 2022].

REPLAS. 2020b. Kapunda Duck Pond Viewing Platform in South Australia Replaced

With Recycled Plastic Decking [Online]. Replas. Available:

https://www.replas.com.au/kapunda-duck-pond-viewing-platform-in-south-

australia-replaced-with-recycled-plastic-decking/ [Accessed 2022].

REPLAS. 2020c. Washing Platforms for Bribie Island Dog Beach Use Recycled Plastic

Decking Enduroplank™ from Replas [Online]. Replas. Available:

https://www.replas.com.au/washing-platforms-for-bribie-island-dog-beach-use-

recycled-plastic-decking-enduroplank-from-replas/ [Accessed 2022].

RHODES, C. J. 2018. Plastic Pollution and Potential Solutions. Science Progress, 101,

207-260.

RIDDLE, T. 2014. Has wood-plastic come of age? Building Connections, 26-28.

RODRIGUES, I. A. P. T. A., R.V.; GUIMARÃES, M.J.D.C.; GOMES, T.S.; PACHECO,

E.B.A.V. 2022. Assessment of plastic lumber production in Brazil as a substitute

for natural wood. Environment, Development and Sustainability 24, 9705–9730.

RUTKOWSKA, M., HEIMOWSKA, A., KRASOWSKA, K. & JANIK, H. 2002.

Biodegradability of polyethylene starch blends in sea water. Polish Journal of

Environmental Studies, 11.

STARK, N. M. Effect of weathering variables on the lightness of high-density

polyethylene woodflour composites. 8th International Conference on Wood-

fiber-Plastic Composites (and other natural fibers), 2005 Madison, Wisconsin,

USA. Forest Products Society, 207-212.

STARK, N. M. 2006. Effect of weathering cycle and manufacturing method on

performance of wood flour and high-density polyethylene composites. Journal of

Applied Polymer Science, 100, 3131-3140.

STARK, N. M., MATUANA, L. M. & CLEMONS, C. M. 2004. Effect of processing

method on surface and weathering characteristics of wood–flour/HDPE

composites. Journal of Applied Polymer Science, 93, 1021-1030.

TAIB, R. M., ZAUZI, N. S. A., ISHAK, Z. A. M. & ROZMAN, H. D. 2010. Effects of

Photo-Stabilizers on the Properties of Recycled High-Density Polyethylene

37

(HDPE)/Wood Flour (WF) Composites Exposed to Natural Weathering.

Malaysian Polymer Journal, 5, 193-203.

UN ENVIRONMENT PROGRAMME 2022. Historic day in the campaign to beat plastic

pollution: Nations commit to develop a legally binding agreement. Nairobi: UN

Environment Programme.

VAN DER VEGT, M., VELZING, E.-J., RIETBERGEN, M. & HUNT, R. 2022.

Understanding Business Requirements for Increasing the Uptake of Recycled

Plastic: A Value Chain Perspective. Recycling, 7, 42.

VAUGHAN LEVITZKE, P. S. M. 2019. The development of a circular economy in

Australia. In: GHOSH, S. K. (ed.) Circular Economy: Global Perspective.

Singapore: Springer.

WILLIAMS, K., CHALMERS, D. & SUORINENI, F. 2015. Assessing the Suitability of

Recycled Material as a Timber Substitute in Drawpoint Design. Journal of

Research Projects Review, 4, 73-82.

WILSON, A. 1999. Recycled Plastic Lumber in the Landscape. Landscape Architecture

Magazine. American Society of Landscape Architects.

XIE, K. Y., LOCKE, D. C., HABIB, D., JUDGE, M. & KRISS, C. 1997. Environmental

chemical impact of recycled plastic timbers used in the Tiffany Street Pier, South

Bronx, New York. Environmental chemical impact of recycled plastic timbers

used in the Tiffany Street Pier, South Bronx, New York, 21, 199-211.

ZHAO, X., KOREY, M., LI, K., COPENHAVER, K., TEKINALP, H., CELIK, S.,

KALAITZIDOU, K., RUAN, R., RAGAUSKAS, A. J. & OZCAN, S. 2022.

Plastic waste upcycling toward a circular economy. Chemical Engineering

Journal, 428, 131928.

ZVEKIC, M., RICHARDS, L. C., TONG, C. C. & KROGH, E. T. 2022. Characterizing

photochemical ageing processes of microplastic materials using multivariate

analysis of infrared spectra. Environmental Science: Processes & Impacts, 24, 52-

61.

38

Appendices

Appendix A:

39