Development and Testing of Compounds Containing Ground Athletic Shoes

TABLE OF CONTENTS

EXECUTIVE SUMMARY................................................................................................ 1

1.0 INTRODUCTION...................................................................................................... 3

1.1 BACKGROUND......................................................................................................... 3

1.2 PROJECT OBJECTIVE............................................................................................... 4

1.3 REPORT ORGANIZATION........................................................................................ 4

2.0 PHASE I, RUBBER CHIP DOOR MATS............................................................... 4

2.1 RUBBER CHIP DOOR-MAT OBJECTIVES.............................................................. 4

2.2 COMPOUNDING RUBBER CHIP DOOR MATS,

MATERIALS AND METHODS.................................................................................. 5

2.2.1 Rubber Chips -- Processing Method.............................................................. 5

2.2.2 Rubber Chip Characteristics.......................................................................... 6

2.2.3 Basic Rubber Compounding.......................................................................... 7

2.2.4 Rubber Chip Door-mat Compounding Procedures......................................... 9

2.3 PERFORMANCE TEST RESULTS FOR DOOR MATS.......................................... 11

2.4 CURING PROPERTIES OF RUBBER CHIP COMPOUNDS.................................. 12

2.5 SMALL SCALE FACTORY TRIALS AND RESULTS............................................ 12

2.6 ECONOMIC FEASIBILITY OF MANUFACTURING RUBBER

CHIP DOOR MATS.................................................................................................. 14

3.0 PHASE II, RUBBER DUST SHOE SOLES.......................................................... 16

3.1 RUBBER DUST SHOE SOLE OBJECTIVES........................................................... 16

3.2 COMPOUNDING RUBBER DUST SHOE SOLES,

MATERIALS AND METHODS................................................................................ 17

3.2.1 Rubber Dust Processing Methods................................................................ 17

3.2.2 Rubber Dust Characteristics......................................................................... 17

3.2.3 Rubber Dust Shoe Sole, Compounding Procedures...................................... 18

3.3 PERFORMANCE TEST RESULTS FOR SHOE SOLES......................................... 21

3.4 CURING PROPERTIES OF RUBBER DUST COMPOUNDS................................. 24

3.5 SMALL SCALE FACTORY TRIALS AND RESULTS............................................ 25

3.6 ECONOMIC FEASIBILITY OF MANUFACTURING RUBBER

DUST SHOE SOLES................................................................................................. 26

4.0 CONCLUSION......................................................................................................... 28

5.0 ACKNOWLEDGEMENTS..................................................................................... 29

APPENDICES

A Akron Rubber Development Laboratory, Inc. Test Results............................................ A

B Smithers Scientific Services, Inc. Abrasion Test Results.................................................. B

C Rheometer Reading for Compounds G and H................................................................ C

D Calculation of Pound Costs for Compounds G-56, G-80, and H-80.............................. D

E Rouse Rubber Industries, Inc., Product Certification....................................................... E

F Smithers Scientific Services, Inc. Abrasion Test Results................................................. F

G Rheometer Readings for Compounds A, A-15, A-25, B, and B-15............................... G

LIST OF TABLES

Table 1 Formulation Ingredients........................................................................................ 10

Table 2 Performance Test Results of Rubber Chip Door Mats.......................................... 11

Table 3 Compound Pound Cost, Specific Gravity, and Material Cost per Mat................... 15

Table 4 Rubber Chip Door-mat Manufacturing Costs....................................................... 16

Table 5 Formulation Ingredients........................................................................................ 20

Table 6 Compound Test Results....................................................................................... 21

Table 7 Rubber Dust Shoe Sole Compound Rheograph Results........................................ 24

Table 8 Batch Weights and Mill Time............................................................................... 26

Table 9 Material Costs for Rubber Dust Compounds........................................................ 27

Table 10 Rubber Dust Shoe Soles Manufacturing Costs...................................................... 28

LIST OF FIGURES

Figure 1 Tensile Strength................................................................................................... 22

Figure 2 Elongation Percentage.......................................................................................... 22

Figure 3 Tear Strength....................................................................................................... 23

Figure 4 Durometer........................................................................................................... 23

Figure 5 Abrasion Index.................................................................................................... 24

EXECUTIVE SUMMARY

Washington State residents dispose of approximately 5 million athletic shoes each year. To date there are very few technologies available to convert the rubber from those shoes into viable products. In recognition of this problem, the Recycling Technology Assistance Partnership (ReTAP) funded a project to assess the viability of manufacturing products from post-consumer athletic shoes.

This report summarizes the results of a technology validation project that evaluated the feasibility of using ground shoe sole rubber to make door mats, and pulverized shoe sole rubber to make shoe soles. The project consists of two phases. Phase I addresses the development, manufacturing, and economics of manufacturing door mats with ground shoe soles. Phase II addresses the development, manufacturing, and economics of manufacturing shoe soles using pulverized shoe soles.

Phase I project results demonstrate that old shoes can be used effectively as a feedstock for compounded door mats. A home door mat was compounded that contained 80% ground post-consumer shoe soles and had the abrasion resistance and hardness to stand up to substantial use. Financial calculations indicate that this mat can be manufactured at a price that would allow it to be retailed for a competitive $28.00.

Phase II project results demonstrate additional potential for using old athletic shoes as a feedstock for shoe soles. A shoe sole was compounded that contained 15% pulverized post-consumer shoe soles and met all of the specifications of two major shoe companies' outsoles. Financial calculations indicate that a pair of these soles can be manufactured for somewhere between $1.42 and $2.38, allowing it to be sold to shoe manufacturers for a competitive price.

Project findings also pointed to some problems when using ground shoe soles in the manufacturing of products. Ground shoe sole material contains small amounts of aluminum (from the eyelets of shoes). This material cannot be removed through magnetic separation, although it might be possible to remove it through an air-separation process. While the aluminum caused no problems in the mat factory trial, many rubber manufacturers steadfastly refuse to use any material containing metal for fear of damaging expensive equipment. When pulverized down to a nominal particle size of #80 mesh, the aluminum posed no problems and was not a concern.

In conclusion, it appears technically viable and economically feasible to create new products using ground and pulverized post-consumer athletic shoes as a feedstock. As a next step, the door mats and shoe soles fabricated through this project need to be field tested.

1.0 INTRODUCTION

This report summarizes the results of a technology assistance project involving the Recycling Technology Assistance Partnership (ReTAP), Carton Environmental Systems, and J. L. Merryfield, Inc., to test the feasibility of incorporating ground and pulverized post-consumer shoe soles into new products.

1.1 BACKGROUND

In Washington, approximately 5.5 million athletic shoes are disposed of each year. Nationally, 270 million shoes are thrown away each year. While some efforts have been made to incorporate athletic shoe sole rubber into playground padding, basketball courts, and running tracks, there is no product on the market to date that contains post-consumer shoe material.

To help develop a market for post-consumer shoe soles, the Clean Washington Center provided funding for this project to develop and test two prototype products made from the soles of post-consumer athletic shoes -- door mats and shoe soles.

HMC Sports, Inc. (HMC Sports ), initiated the project in order to find a use for the shoes collected from its customers. HMC Sports operates eight athletic shoe stores throughout the Northwest and gives customers a discount on new shoes when they bring in a pair of used shoes. HMC Sports began investigating shoe recycling on its own and found that a rotary knife granulator built for processing tires could be used to turn shoes into approximately a #14-mesh particle size. HMC Sports also found that air-separating equipment traditionally used to separate the rubber and fabric portions of tires could be used to separate the soles and uppers of athletic shoes.

Through further work, HMC Sports found that Rouse Rubber of Vicksburg, Mississippi, had developed a proprietary, ambient temperature, wet-grind technology to pulverize rubber chips into a nominal #80-mesh particle size.

1.2 PROJECT OBJECTIVE

The objective of this project was to determine the feasibility of incorporating rubber chips into door mats and rubber dust into shoe soles. To meet this objective, rubber chips and rubber dust were compounded into laboratory samples, formulated into door mats and shoe soles, and factory trials conducted. Testing was also undertaken to determine the properties of the resulting compounded products.

1.3 REPORT ORGANIZATION

This report is organized into two main sections. The first addresses the door mats and the second addresses the shoe soles. Each section reviews the methods used to formulate test results, factory trial procedures and findings, and estimates of manufacturing costs.

2.0 PHASE I, RUBBER CHIP DOOR MATS

2.1 RUBBER CHIP DOOR-MAT OBJECTIVES

The objective of Phase I was to develop door mats containing pulverized rubber chips that could be manufactured for a target cost of $7.00. This manufacturing cost would allow the product to come to market for a wholesale cost of $14.00 and a retail cost of $28.00, making it competitive with other decorative home door mats.

Performance specifications are not available for traditional decorative home door mats. J. L. Merryfield, Inc., the rubber expert involved with this project, developed the following specifications for the doormats:

· abrasion resistance index of 70% per ASTM D 1630 to ensure their ability to withstand use, and

· durometer (hardness) of 70 A per ASTM D 2240 to give them a sturdy, firm feel.

The selection of these performance characteristics was somewhat arbitrary. If a softer mat had been desired, a lower durometer could have been specified.

It should be noted that while laboratory tests can give an indication of how a product will perform its actual function, they are far from conclusive. A product that performs well in laboratory tests will not necessarily perform well in the field. Generally, however, if a product fails performance tests, it will also perform poorly in the field.

Because other rubber professionals might be interested in using rubber chips in a variety of other applications, the project team conducted tests to measure tensile strength, elongation, specific gravity, tear strength, and durometer. While of interest, these properties do not significantly affect the performance of a door mat. Testing was also conducted to learn about the cure time, minimum torque, maximum torque, and scorch time of the mat formulation.

2.2 COMPOUNDING RUBBER CHIP DOOR MATS, MATERIALS AND METHODS

2.2.1 Rubber Chips -- Processing Method

The rubber chips were made from the athletic shoes HMC Sports collected through its retail shoe stores. A group of five laborers sorted the shoes as they were loaded for trucking to Rubber Granulators, Inc. Rubber Granulators ground the shoes with a rotary knife granulator used primarily for tires. The sorters removed all shoes that contained mercury light switches (the mercury in these switches is hazardous) and hiking boots containing steel shanks (steel shanks have the potential to damage grinding equipment).

In granulating the shoes, Rubber Granulators found that by volume, the shoes were easier to grind than tires because they were not as tough. However, because shoes are less dense than tires, by volume, they were slower to grind. Ferrous metals were removed using a magnetic drum. Rubber Granulators charged $0.15 per pound for chipping and air separating the shoes.

2.2.2 Rubber Chip Characteristics

Akron Rubber Development Laboratories, the Akron, Ohio, firm responsible for material characterization, found that most rubber chips fell between the #10- and #14-mesh sizes. Sieve analyses (ASTM D 5644) results are as follows:

Sieve Analysis

Instrument: RO-TAP Shaker

Sieves Used: #10, #14, #40 (United States Standard)

Shaking time: 30 minutes

Retained on #10-mesh sieve: 86.1%

Retained on #14-mesh sieve: 10.6%

Retained on #20-mesh sieve: 2.3%

Passed through #20-mesh sieve: 1.0%

Passed through #40-mesh sieve: 0.0%

Akron also measured fiber content by weight per ASTM D 297 and found it to be 9.6% (Appendix A). In general, chip moisture was not measured, although a pulverized version of the chips was measured and moisture was found to be 1.53%.^[1] Excessive moisture (3% or more) in a rubber compound is likely to cause problems ranging from interfering with the cure system to causing blowing or sponging. A moisture content of 1.53% will pose no problems and can actually enhance surface activity.

Through the course of formulating compounds with the rubber chips, it was found that they contained small flecks of aluminum that came from shoe eyelets. Further work is needed to see if possible to remove the aluminum through adjustments to the air-separation system.

In order to explain how ingredients for the formulations were selected, a brief discussion on the basics of rubber compounding follows.

2.2.3 Basic Rubber Compounding

In general, the ingredients of a rubber compound may be divided into five groups. This statement is, by necessity, an oversimplification and is intended as a guide only. Some ingredients in any given formulation might serve two, or even three purposes. Also, any given ingredient might function differently in various elastomers. With these caveats in mind, the following represent the standard ingredients of a rubber compound.

1. Base Elastomer: There are approximately twenty-two commercially available polymers in the U.S.A. A base polymer, or sometimes a blend of polymers, is chosen based on its cured characteristics: abrasion resistance, oil resistance, high and low temperature characteristics, etc. Most always, the over-riding consideration is cost.

2. Fillers and Plasticizers: Most rubber formulations would be useless without some type of filler incorporated. Fillers can be grouped into three categories: reinforcing, semi-reinforcing, and extenders.

A. Reinforcing fillers include the small particle-sized carbon blacks such as

N110-0, N220, N330 and light-colored reinforcing agents such as Hisil and Cabosil.

B. Semi-reinforcing fillers include black carbon with larger particle sizes such as N762 and N990. Examples of semi-reinforcing fillers include treated clays and zeolex.

C. Extending fillers are often necessary to meet high-quality specifications such as soft printing rolls where a low durometer and resistance to the inks solvents are required.

Plasticizers: Generally plasticizers are liquids or very soft semi-solids. Theymust be carefully chosen on the basis of their compatibility with the elastomer. Those that are incompatible can interfere with the curing systems, leach out after a period of time, or both. In products that must be bonded, plasticizer choice often depends upon individual accelerated, long-term testing, or both.

3. Antidegradents or Protective System: Anti-oxidants and anti-ozonants are generally incorporated from 1.0 to 2.0 parts per 100 parts base elastomer. The chemistry of reactions, whether intended or unintended, is complex. Choosing a compatible antidegradant that will not "bloom" often requires individual compound testing.

4. Special Ingredients: Often ingredients are required for a specific purpose. Examples include flame retardants, coloring agents, and process aides. Ingredients such as PEG 3350 are used to overcome the effects of light-colored fillers. The types and amounts of ingredients used vary considerably.

5. Curing Systems: First, we must assume that all of the above ingredients have been chosen so as not to interfere with cross-linking. Second, we will assume a sulfur-based (not dicumyl peroxide nor radiation) curing.

A. The activators: Generally the combination of zinc oxide and stearic acid work together to provide suitable conditions for curing to take place.

B. Acceleration Systems: Any combination of a host of generally organic compounds that dramatically reduce sulfur cross-linking time.

C. Sulfur: The cross-linking agent.

2.2.4 Rubber Chip Door-mat Compounding Procedures

A total of five door-mat compounds were developed. Two were controls and contained no rubber chips. The other three were experimental compounds and contained as much as 80% rubber chips. This 80% target was arbitrarily chosen at the outset of the project. The compounds developed included:

· G (a control),

· H (a control), identical to G but containing Vestenamer 8012,

· G-56 -- compound G with rubber chips added to make up 56% of the total mixture,

· G-80 -- compound G with rubber chips added to make up 80% of the total mixture, and

· H-80 -- compound H with rubber chips added to make up 80% of the total mixture.

For this project, the ingredient groupings for compound H included:

1. Base Elastomer:

· SBR 1502

· Vestanamer 8012

2. Fillers and Plasticizers:

· Hisil 233

· Cyclolube Napthenic Process Oil

3. Antidegradents or Protective system:

· Agerite Stalite S

4. Special Ingredients:

· Peg 3350

5. Curing System:

· Accelerators

(ALTAX), MBTS, (Methyl Tuads) TMTD

· Activators

Zinc Oxide,

Stearic Acid

· Cross Linking Agent

Sulfur

The ingredients of the control formulations and three variations are listed in Table 1. Also listed in Table 1 is the rationale behind content selections.

Table 1 Formulation Ingredients
	Parts per 100 parts rubber
Ingredient and Rationale	G	G-56			G-80		H		H80
SBR 1502 -- Least expensive of the elastomers. Coagulated with fatty acid and therefore less likely to become sticky	100.0		100.0	100.0		80.0		80.0
Vestenamer 8012 -- provides continuity to the highly chip extended formulation. Vestenamer 8012 is a polyoctnamer rubber and acts as a cross-linkable process aid. It also enhances abrasion resistance.	xxx		xxx	xxx		20.0		20.0
Hisil 233 -- a highly reinforcing light-colored filler	30.0		30.0	30.0		30.0		30.0
Zinc Oxide -- part of the accelerator activation system	5.0		5.0	5.0		5.0		5.0
Stearic Acid -- part of the accelerator activation system.	2.0		2.0	2.0		2.0		2.0
Agerite Stalite, S (high molecular weight glycol) -- a non-staining anti oxidant that prevents cracking and degradation. It also provides a more predictable cure curve in the presence of light-colored fillers and moisture than compounds without it.	1.0		1.0	1.0		1.0		1.0
PEG 3350 -- provides reliability and stability to the curing mechanism under factory conditions. It is one of the least expensive additives used with light color fillers such as Hisil 233.	2.0		2.0	2.0		2.0		2.0
MBTS Altax -- part of the acceleration system.	1.0		1.0	1.0		1.0		1.0