Paper No. 29

“A new polyester bicomponent fiber that allows

a solvent free treating for power transmission belts”

by Elke Gebauer* and Dan Gajewski

KOSA

Highway 70 West

Salisbury, NC 28145

Presented at a meeting of the

Rubber Division, American Chemical Society

Nashville Tennessee

Sept. 29 - Oct. 2, 1998

*speaker

Paper No. 29

Abstract:

A new polyester bicomponent fiberthat allows a solvent free treating for power transmission belts

KOSA introduces a unique new fiber, composed of an HMLS (high modulus, lowshrinkage) polyester core surrounded by a fusible PBT sheath. This new fiber can be plied andtwisted into a high performance cord suitable for use in power transmission belts. During the cordtreating process, the sheath melts and fuses to achieve the filament bonding necessary for cutedge belts. Conventional organic solvents are not required. This solvent elimination allows fiberconverters to meet strict Clean Air Act guidelines without the high cost of organic solventcontainment. Fiber data, treating recommendations, and cord performance data is presented.Extension of this technology to other end uses is discussed.

A new polyester bicomponent fiber that allows a solvent free treating

for power transmission belts

Introduction

Since their first appearance, power transmission belts have passed through a long evolution [l].

In the beginning, developments were driven by the need for higher power transmission (Fig. 1).

New materials on the reinforcement and elastomer side improved belt performance. Starting with

cotton as the first available fiber, then moving to nylon, polyester (PET) has now become the

preferred reinforcement material for power transmission belts [2]. The introduction of high

modulus, low shrinkage (HMLS) PET, with its’ outstanding dimensional stability, increased belt

life and performance further. The change to cut edge belts was important.

In recent years, the automotive industry has become the driving force for belt development. Top

priority has been cost performing belts.

Now, a new driver forces belt companies in the U.S. to reconsider their belt manufacturing for

the next century. This new driver is the Clean Air Act 2000, enforced by the U.S. Environmental

Protection Agency. Power transmission belt manufacturers in the U.S. are impacted through their

coating process, the fiber treatment. Cut edge transmission belts, either V-belts or multi-V belts

(Fig. 2) require stiff treated cord. To accomplish this, the cord is treated with a solvent

(commonly toluene) plus isocyanate solution. Organic solvent emissions will come under

restriction when the Clean Air Act is enforced.

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Clean Air Act impact on belt manufacturers (Fig. 3)

The Clean Air Regulation, which was passed in 1990, has a goal to reduce the emission of

pollutants in the U.S. It is part 2 of the regulation that affects volatile organic compounds (VOC)

emitted into the air. The Federal Government issued a VOC list. Companies that are handling

listed VOC’s already need a state permit for their existing operations. It allows them to operate

without any changes until the final Clean Air Act has been approved. All new equipment,

however, has to comply immediately with the year 2000 requirements.

Part 3 of the regulation defines the MACT-standard, which stands for maximal achievable control

technology. Belt manufacturers will fall under the MACT standard for surface coating. It will

describe necessary actions and the control technology to verify Clean Air compliance if VOC’s

are still in use.

This means a major competitive disadvantage for the U.S. belt manufacturers in a time of

globalization and business concentration. They have to determine, how to meet these upcoming

strict regulations.

Options to meet the Clean Air Act Guidlines (Fig. 4)

Solvent containment and incineration is one possible answer to the Clean Air Act. It allows use of

existing treating technology. Equipment is available, but at a high cost. There have been attempts

to reduce or even eliminate the solvent used, or to change to less critical one. The application of

water based resins as epoxy, melamine or urethane has been tried. None has proven to be as good

as the solvent/isocyanate system.

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A possible consideration is to outsource the treating process. Converting capacity is available in

the global market. European belt manufacturers have taken this path. However, unlike Europe,

where independent converters provide treated cord, the number of converting companies for V-

belt stiff cord is limited in the U.S. A European supply would add transportation and duty costs.

But more important, outsourcing would mean giving up know how that has contributed to the

world market leadership for U.S. belt manufacturers.

A new polyester fiber solution

KOSA has always been dedicated to its’ customers, in providing knowledge and technology

support. HMLS PET, which had allowed very significant improvements in technical applications

such as tires, V-belts, and hoses, was invented by KOSA (Hoechst Celanese). Now, we present a

development to the industry which could revolutionize the stiff cord treating process by offering

a PET fiber to meet the Clean Air Act guidelines. This new fiber is KOSA 796 (Fig. 5). It is a

polyester bicomponent or heterofil fiber composed of two different PET polymers in a core-

sheath arrangement. The filament core is extruded of polyethylene terephthalate (PET), and is

surrounded by a sheath of polybutylene terephthalate (PBT). PBT has a melting point of about

225°C. PET melts at approx. 256°C. Heterofil has an adjustable core sheath ratio. For V-belt stiff

cord applications, it is 90% core and 10% sheath.

Bicomponent fibers are not a new invention. The first bicomponent fibers were commercially

available in the mid 60’s. Today, they are widely used as textile and staple fibers [3].

Over the last few years, we developed technology to manufacture PET bicomponent

multifilament fibers especially engineered for the power transmission belt.

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Heterofil yarn and cord requirements for belt reinforcement

1. Adhesion of core to sheath (Fig. 6)

Finding a suitable sheath polymer is important for two reasons. Good core-to-sheath adhesion is

necessary for the fiber spinning process, and to ensure the integrity of the treated cord bundle.

This adhesion has to exist in the unfused and fused state. Fig. 7 and Fig. 8 show cross sections of

heterofil yarn and a heat set cord. The surrounding of the PET core, by the PBT sheath, is clearly

visible.

2. HMLS fiber character (Fig. 9)

Since HMLS PET has proven to be superior to conventional PET fibers in belt performance it

became the basis for heterofil development. Dimensional stability is responsible for the favorable

creep growth and good tension retention of HMLS, which has improved belt life [4].

Dimensional stability is the sum of elongation at a specific load and hot air shrinkage(Fig. 10). A

material is more dimensional stable when the sum is small. It is common to use a dimensional

stability rating (DSR) to compare fibers. The dimensional stability of the control fiber is divided

by that of the compared fiber. A value greater one indicates a better DSR than the control. HMLS

achieves its desirable dimensional stability during treating [5].Heterofil yarn and cord properties

are given in Fig. 11 and Fig. 12 in comparison to HMLS types. The dimensional stability of

treated heterofil cord is equal to or superior to other I-IMLS types.

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3. Processability (Fig. 13)

An important requirement for heterofil is to perform on existing process equipment used to

manufacture V-belt stiff cord. The stiff cord process can be divided into two steps; 1) a textile

processing step and, 2) a chemical/thermal step.

The textile step comprises two phases; 1) single yarn twisting and, 2) cord cabling. Heterofil

performs on industrial twisters and cablers comparably to PET homotilament yarns.

In the following chemical/thermal treating step, heterotil shows its uniqueness.

Regular PET cord is converted into stiff cord in two stages (Fig. 14). A solvent/isocyanate

solution is applied in the first zone. The isocyanate penetrates into the fiber bundle, reacting

either with the PET carboxyl end groups or the reactive finish groups on the fiber. Under the

applied temperature, the solvent evaporates. The isocyanate bonds the filaments together, cross-

linking to a stiff network. The isocyanate also reacts with the secondly applied resorcinol

formaldehyde latex (RFL). The result is a bonded stiff cord that adheres well to rubber

compounds. In an optional third zone, a post cement (generally dissolved rubber and a tackifier)

can be applied to improve cord tack. Today, it is common practice to operate treating units

without control of emitted volatile organic gasses.

The concept behind heterofil is to melt the sheath during cord treating. In this phase it flows

within the spaces of the single filaments and bonds them together. As the cord cools below the

melting point of PBT, it stiffens (Fig. 15). Existing treating units capable of temperatures of at

least 230°C in at least one oven are suitable for heterofil treating. It is possible to apply the

necessary fusing temperature in either the first or second zone. The treating alternative chosen

affects the dip recipe, which will be discussed in more details later. The application of a post

cement in a third zone is optional, as it is for standard PET.

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4. Self-stiffening solvent free (Fig. 16)

Unlike standard PET, heterofil self stiffens without any organic solvent. The stiffness of treated

heterofil cords is a measure of the PBT fusing and the filament bonding. Adequate bonding of the

cords, in raw edge V and multi-V belts, is crucial to belt performance. Belt failure in service

occurs when the edge cord pops out, and starts fraying. Eventually, the cord gets pulled out of the

rubber, and the belt fails. Optimal fusing of the sheath is necessary to obtain good filament

bonding. In the lab we test bending stiffness to verify filament bonding(Fig. 17). A treated cord

is placed horizontally in a moveable pull rod. Then it is pulled vertically through a hole in a metal

bar. The stiffer the cord, the higher the force needed to bend and to pull it through the hole.

A study with heterotil cords was conducted, using a standard V-belt construction (1100/2/3

dtex*). The purpose was to determine the operating window for optimal stiffness in treating. In

an experimental design using treating alternative 1 for heterofil (Fig. 15) temperature and dwell

time in zone 2 were varied. Fig. 18 illustrates that the temperature was the more significant factor

influencing stiffness. The response surface shows a maximum at 235 to 238°C. At those

temperatures dwell time plays only a minor factor. The fusing of the PBT happens fairly quickly,

since it is a physical transformation. Dwell time should be increased if lower temperatures are

used, to allow the heat to penetrate into the cord interior. Since the necessary time to fuse the

sheath is shorter than the drying or curing time of the RFL, heterolil does not extend the treating

process.

5. Adhesion to standard elastomers (Fig. 19)

In a standard stiff cord treatment, the applied isocyanate has several functions.

It bonds the filaments; it stiffens the cord by forming a three dimensional network; and it acts as

an adhesive agent between PET and resorcinol formaldehyde resin.

* dtex = g/l 0,000 m- 6 -

Polymeric methylene diphenyl diisocyanate (MDI) is commonly used as first dip for standard

stiff cord. It is also available in blocked form. In this case the isocyanate groups have been

reacted with other molecules. Blocked isocyanates are generally mixed into the dip in powder

form, or as an aqueous dispersion. They are widely used for tire cord or conveyor belt fabric

treatment. Examples of blocked isocyanates are LVBITM1 and Grilbond IL6TM2. The adhesion

mechanism is based upon the unblocking of the isocyanate groups in heat. They are less reactive

than the polymeric MDI, and adhesive activated PET yarn types are preferable. In general, PET,

due to its lack of functionality, needs a special surface activation for rubber application. The

activation increases the surface reactivity, promoting adhesion.

Heterotil is available with surface activation, achieved via application of an adhesive activated

finish. Grilbond IL6 was chosen as a dip adhesion promoter for heterofil treating. It is a

caprolactam blocked isocyanate, releasing caprolactam when unblocking during the treating step.

As previously mentioned, heterotil leaves two treating alternatives, due to its unique self-

stiffening property. Alternative 1 uses a lower first zone temperature and requires the high

temperature to fuse the PBT sheath in the second zone. Alternative 2 applies the high temperature

in the first zone, followed by a lower temperature in the second zone. This option allows the use

of dip ingredients that are heat sensitive, such as those that cross link at high temperatures. Each

treating alternative opens two dip options, shown in Fig. 20. An adhesion comparison to standard

stiff cord is given in Fig. 21.

Heterofil provides comparable adhesion to standard rubbers, e.g. styrene-butadiene rubber (SBR)

and chloroprene rubber (CR), giving greater flexibility in treating.

We believe that the PBT sheath plays a key role in the heterofil adhesion mechanism.

1 Uniroyal Chemical Co, Middlebury, CT 067492 Ems American Grilon, 2060 Corporate Way, Sumter, SC 29 15 1- 17 17

- 7 -

It carries the adhesive activated finish, with which the unblocked isocyanate groups react during

treating. Low crystallinity facilitates this by promoting infiltration of molecules, such as finish

components or isocyanate groups, into its amorphous structure.

6. Fatigue Resistance (Fig. 22)

A power transmission belt endures compression and extension cycles in service. The fatigue

resistance of the reinforcing fiber, the ability to withstand the cycling load, has an important

impact on belt life. Though PET possesses good fatigue properties, there are notable differences

between conventional and HMLS PET. The improved performance of HMLS is believed to be

related to its higher toughness.

We evaluated the fatigue resistance of treated heterofil cords in comparison to HMLS stiff cords

on a shoe shine tester (Fig. 23). 25.4 mm wide rubber pads are prepared containing 20 ends of

1100/2/3 dtex treated cords. The pads are installed by bending them over a 15 mm diameter

spindle. The ends are fixed in clamps, and the pads are loaded. During testing, the pads move up

and down over the spindle, with a stroke of 120 mm and a frequency of 2 cycles per second. This

movement simulates the tension and compression of the cords in a belt operating around pulleys.

The fatigue resistance of the fiber is the ratio of strength retained after a defined number of cycles

to initial cord strength. The fatigue results of regular HMLS stiff cord and heterofil are shown in

Fig 24. The superior performance of heterofil is evident.

The standard system has, due to the cross linked isocyanate, a fairly high modulus. The stiff,

brittle structure restricts fiber movement, leading to inferior fatigue resistance.

Heterotil, with its lower modulus fused PBT sheath, is capable of damping the cycling load, and

protecting the PET fiber better from being damaged.

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7. Environmental friendly Treatment (Fig. 25)

It was mentioned earlier that existing treating units for stiff cord in the U.S. have almost no

solvent containment, emitting volatile gasses directly into the air. To define the volume of these

gasses, we executed an experiment on our single end Litzler ComputreaterTM3 (Fig. 26). We ran

a HMLS 1100/2/3 dtex cord through a 10% isocyanate/toluene solution for 10 min., and dried it

in the first zone oven. By measuring the cord weight before and after treating and the remaining

isocyanate/solvent bath, a ratio of consumed greige cord to emitted solvent of 1:0.3 was

determined, nearly one third.

Projecting this information to a one shift production unit of 40 ends with a speed of 30 to 40

m/min., we calculated emissions of 29 to 38 tons of volatile gases (Fig. 27).

With the upcoming Clean Air Act this volume has to be reduced nearly 100%. Additionally, the

Act will require manufacturers to install surveillance technology on their production sites.

Altogether the solvent will be paid for three times. Thus the heightened interest in new stiff cord

production alternatives.

Future possible applications of heterofil (Fig. 28)

So far, we have focused our work on power transmission belts. But the heterotil concept holds

many other possibilities. Heterofil could be useful as a monofilament substitution. Its “monofil”

character, after the sheath is fused, could be interesting for hose or light conveyor belt. Heterofil

provides better adhesion to elastomers and thermoplastics than monofilament. Further ideas

include snow mobile track reinforcement or chafer fabric.

3 C.A. Litzler Co., Inc., 4800 W. 160 St., Cleveland, Ohio 44135-2689- 9 -

Other sheath polymers can broaden the field of applications. Right now, we are working with

three different sheath polymers: polybutylene terephthalate (PBT), polyethylene (PE) and

polypropylene (PP). The polyolefins certainly offer additional opportunities. Some applications

under development are dental floss, geogrid, sail cloths or inner liners.

We believe that KOSA 796, with its unique properties, has greater potential than we have defined

to date. It can offer intelligent solutions, heterofil engineered solutions.

Acknowledgment

The author wishes to thank the lab technicians Mark Johannson and Jim Motley for their help and

the time they dedicated to this presentation. Many thanks also to Dan Gajewski for being co-

author of this presentation.

References

[l] Fukuda, M., Shioyama, T., Mikami, Y., V-belt and Fan belt manufacturing technology, in

Rubber Products Manufacturing Technology, New York, Basel, Hong Kong, pp 593-649

[2] Stanhope, H., V-belt reinforcement- Polyester, the most popular fiber, presentation at the

125th meeting of the Rubber Division of the American Chemical Society

[3] Davies, B, Advanced Heterofil fiber technology and applications, HCC

[4] Leumer, G., Gebauer, E., Schaefer, R., Hochfeste Polyestermultifilamente als

Verstaerkungsmaterial in Antriebsriemen, in KGK Kautschuk Gummi Kunststoffe, Nr. 3/97, pp

198-207

[5] Leumer,G., Roetgers, A., High Modulus Low Shrinkage- Polyester Multifilamente als

Verstaerkungsmaterial in der Reifenkarkasse, in KGK Kautschuk Gummi Kunststoffe, Nr. l/95,

pp 22-28

- l O -

I

Drivers ofPower Transmission Belt Developments

l Improved Power Transmission@improved raw materials, fibers and elastomers

l Improved Belt Life advanced PET fibers, HMLS PET

*improved belt design, raw edge belts, Multi-V belts

l Improved Cost / Performance better performing elastomers, CR, ACSM, HNBR,

EPDM

l Compliance with the Clean Air 2000 ?

- l l -

Heterofil single filaments

PBT Sheath

PET Core

Fig. 7

Fig. 8

Requirements of Heterofil

Fig. 9

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Yarns Properties of

Adhesive Activated

dtex = g/10,000 mFig. 11

Treated Cord properties of Heterofiland KOSA HMLS Fibers

*TPM = turns per meter

Fig. 12

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