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                                                                    EXHIBIT 4-AG





                          SIXTH SUPPLEMENTAL INDENTURE

                                     between

                             HERCULES INCORPORATED,
                                    as Issuer

                                       and

                            THE CHASE MANHATTAN BANK,
                                   as Trustee

                          Dated as of February 9, 2000
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                                TABLE OF CONTENTS
                             ----------------------

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ARTICLE 1

SECTION 1.01.  Amended Definitions.......................................      2
SECTION 1.02.  Additional Definitions....................................      2
SECTION 1.03.  Section 1.01(h)...........................................      3
SECTION 1.04.  Deleted Definition........................................      3
SECTION 1.05.  Amendment of the Subordinated Notes.......................      3
SECTION 1.06.  Application of Articles 1, 2 and 3........................      3

ARTICLE 2

SECTION 2.01.  Amendment of Interest Rate Calculation on the
         Subordinated Notes..............................................      3

ARTICLE 3

SECTION 3.01.  Remarketing...............................................      7

ARTICLE 4

SECTION 4.01.  Ratification of Base Indenture, the First Supplemental
         Indenture, the Second Supplemental Indenture, the Third
         Supplemental Indenture and the Fifth Supplemental Indenture:
         Sixth Supplemental Indenture Controls...........................     11
SECTION 4.02.  Trustee Not Responsible for Recitals......................     12
SECTION 4.03.  Governing Law.............................................     12
SECTION 4.04.  Severability..............................................     12
SECTION 4.05.  Counterparts..............................................     12
SECTION 4.06.  Terms Defined.............................................     12
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         SIXTH SUPPLEMENTAL INDENTURE, dated as of February 9, 2000 (the "SIXTH
SUPPLEMENTAL INDENTURE"), between Hercules Incorporated, a Delaware corporation,
as issuer (the "COMPANY"), and The Chase Manhattan Bank, a New York banking
corporation, as trustee (the "TRUSTEE").

         WHEREAS, the Company and the Trustee are parties to the Junior
Subordinated Debentures Indenture dated as of November 12, 1998 between the
Company and the Trustee (the "BASE INDENTURE"), as supplemented by a First
Supplemental Indenture dated as of November 12, 1998 between the Company and the
Trustee (the "FIRST SUPPLEMENTAL INDENTURE"), a Second Supplemental Indenture
dated as of July 6, 1999 (the "SECOND SUPPLEMENTAL INDENTURE"), a Third
Supplemental Indenture dated as of October 25, 1999 (the "THIRD SUPPLEMENTAL
INDENTURE"), a Fifth Supplemental Indenture dated as of January 24, 2000 (the
"FIFTH SUPPLEMENTAL INDENTURE" and together with the Base Indenture, the First
Supplemental Indenture, the Second Supplemental Indenture, the Third
Supplemental Indenture and this Sixth Supplemental Indenture, the "INDENTURE");

         WHEREAS, the Company executed and delivered the Base Indenture to the
Trustee to provide for the issuance of the Company's unsecured junior
subordinated debentures (the "DEBENTURES") to be issued from time to time in one
or more series as might be determined by the Company under the Indenture, in an
unlimited aggregate principal amount which may be authenticated and delivered as
provided in the Base Indenture;

         WHEREAS, pursuant to the terms of the Indenture in the First
Supplemental Indenture, the Company provided for the establishment of a new
series of its Debentures known as its Auction Rate Reset Junior Subordinated
Notes Series A (the "SUBORDINATED NOTES");

         WHEREAS, the Indenture provides that the Company and the Trustee may
amend the Indenture, with the consent of each Holder of any Debenture affected
thereby, to provide for, among other things, changes in the stated maturity and
interest rate of that series of Debentures;

         WHEREAS, the Company and the Trustee desire to modify certain
provisions of the Indenture to extend the maturity date and change the interest
rate of the Subordinated Notes and to provide for remarketing of the
Subordinated Notes in certain circumstances;

         WHEREAS, all things necessary to make this Sixth Supplemental Indenture
a valid indenture and agreement according to its terms have been done;
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         NOW THEREFORE, in consideration of the purchase and acceptance of
the Subordinated Notes by the Holder thereof, and for the purpose of amending
and restating certain terms of the Indenture relating to the stated maturity,
interest rate and remarketing of the Subordinated Notes, the Company covenants
and agrees with the Trustee as follows:



                                    ARTICLE 1


         SECTION 1.01.  Amended Definitions.  The definition of "Maturity Date"
contained in Section 1.01 of the Second Supplemental Indenture is hereby
amended to read in its entirety as follows:

                  ""MATURITY DATE" means February 9, 2002; provided, that in the
         event of a successful Remarketing of the Subordinated Notes or the
         Preferred Securities, as the case may be, the Maturity Date shall be
         the Remarketed Redemption Date; and provided, further, that, in
         accordance with Section 10.02(a)(iv), in the event the Company elects
         to pay the outstanding principal of and accrued and unpaid interest on
         the Subordinated Notes upon receipt of the Remarketing Notice, and
         notifies the Remarketing Agent of such election within five Business
         Days thereafter, the Maturity Date shall be the date eight Business
         Days after receipt of the Remarketing Notice."

         SECTION 1.02. Additional Definitions. Section 1.01 of the First
Supplemental Indenture is hereby amended by inserting the following definitions
in the appropriate alphabetical order:

                  ""AFFILIATED BIDDER" has the meaning set forth in Section
10.02(b)."

                  ""DATE OF DETERMINATION" has the meaning set forth in Section
2.05(b)."

                  ""PRE-REMARKETING INTEREST PAYMENT DATE" has the meaning set
         forth in Section 2.05(b)."

                  ""PRE-REMARKETING REGULAR RECORD DATE" has the meaning set
         forth in Section 2.05(c)."


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<PAGE>   5
                  ""REMARKETED REDEMPTION DATE" means the later of (i) the first
anniversary of the Remarketing Settlement Date on which Replacement Securities
are issued, and (ii) February 9, 2002."

                  ""REMARKETING NOTICE" has the meaning set forth in Section
10.02(a)(i)."

                  ""REQUESTING HOLDERS" has the meaning set forth in Section
10.02(a)(i)."

         SECTION 1.03. Section 1.01(h). The term "ASSOCIATED PERSON" is hereby
added to the list of defined terms contained in Section 1.01(h) of the First
Supplemental Indenture.

         SECTION 1.04.  Deleted Definition.  The term "MATURITY EXTENSION DATE"
is hereby deleted in its entirety from Section 1.01(h) of the First Supplemental
Indenture.

         SECTION 1.05. Amendment of the Subordinated Notes. The Company shall
execute replacement Subordinated Notes in the form attached hereto as Exhibit A
to reflect the amended terms provided for in this Sixth Supplemental Indenture,
and the Trustee shall authenticate and make such new Subordinated Notes
available for delivery to the Holders of the Subordinated Notes upon surrender
of the prior certificates therefor. The surrendered prior certificates
representing the Subordinated Notes shall be canceled by the Trustee and shall
no longer be outstanding.

         SECTION 1.06. Application of Articles 1, 2 and 3. The provisions of
Article 1, Article 2 and Article 3 hereof shall apply to the Subordinated Notes
and the certificates therefor shall be appropriately amended.

                                    ARTICLE 2


         SECTION 2.01.  Amendment of Interest Rate Calculation on the
Subordinated Notes.  Section 2.05 of the First Supplemental Indenture is hereby
amended to read in its entirety as follows:

                  "SECTION 2.05. Interest. (a) Interest on the principal amount
         of each Subordinated Note will accrue and be payable at a rate (the
         "INTEREST RATE") per annum equal to

                       (i) from and including November 12, 1998 to but excluding
                  February 9, 2000, LIBOR plus 175 basis points;


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<PAGE>   6
                      (ii) from and including February 9, 2000 to but excluding
                  the earlier of (A) the Remarketing Settlement Date on which
                  Replacement Notes are issued and (B) the date such principal
                  amount is paid, LIBOR plus 150 basis points;

                     (iii) from and including the Remarketing Settlement Date on
                  which Replacement Notes are issued to but excluding the date
                  such principal amount is paid, the Winning Bid Rate; and

                      (iv) notwithstanding clauses (i), (ii) and (iii) above, if
                  the Company fails to pay the principal amount on the date such
                  amount becomes due, then from and including such due date to
                  but excluding the date such principal amount is paid, the
                  applicable Interest Rate in effect on the due date, compounded
                  quarterly, but only to the extent permitted by applicable law.

                  Interest that is not paid when due will bear additional
         interest thereon compounded quarterly at the applicable Interest Rate
         in effect on the due date specified above (to the extent permitted by
         applicable law). The term "INTEREST", as used herein, includes any such
         additional interest unless otherwise stated.

          (b) Until the Remarketing Settlement Date on which Replacement Notes
         are issued, interest on the Subordinated Notes will be payable
         quarterly in arrears (A) on February 12, May 12, August 12 and November
         12 of each year, commencing February 12, 1999 and (B) on such
         Remarketing Settlement Date (each, a "PRE-REMARKETING INTEREST PAYMENT
         DATE"), and will accrue from and including the most recent date to
         which interest has been paid or, if no interest has been paid, from
         November 12, 1998, to but excluding the related Pre-Remarketing
         Interest Payment Date, except as otherwise described below.

                  The Interest Rate in effect for the period from and including
         November 12, 1998 to but excluding February 12, 1999 shall be the rate
         determined by the Calculation Agent two London Banking Days prior to
         November 12, 1998 and shall equal LIBOR plus 175 basis points. The
         Interest Rate in effect from and including February 12, 1999 to but
         excluding February 9, 2000, for each quarterly period from and
         including the immediately preceding Pre-Remarketing Interest Payment
         Date to but excluding the applicable Pre-Remarketing Interest Payment
         Date, shall be determined by the Calculation Agent two London Banking
         Days prior to such immediately preceding Pre-Remarketing Interest
         Payment Date (a "DATE OF DETERMINATION") and shall equal LIBOR plus 175
         basis points.


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         The Interest Rate in effect thereafter, for each quarterly period from
         and including the immediately preceding Pre-Remarketing Interest
         Payment Date to but excluding the applicable Pre-Remarketing Interest
         Payment Date, shall be determined by the Calculation Agent two London
         Banking Days prior to such immediately preceding Pre-Remarketing
         Interest Payment Date and shall equal LIBOR plus 150 basis points. The
         amount of interest payable on February 12, 2000 shall reflect the pro
         rata application of the two Interest Rates applicable to the
         calculation period for such interest payment for each day on which the
         applicable Interest Rate was in effect. The amount of interest payable
         for any quarterly period shall be computed on the basis of a 360-day
         year of twelve 30-day months. Except as provided in the last sentence
         of this paragraph, the amount of interest payable for any period
         shorter than a full quarterly period for which interest is computed
         will be computed on the basis of the actual number of days elapsed per
         30-day month. If a Pre-Remarketing Interest Payment Date is not a
         Business Day, then such Pre-Remarketing Interest Payment Date will be
         the next succeeding Business Day, except if such Business Day is in the
         next succeeding calendar month, such Pre-Remarketing Distribution Date
         will be the immediately preceding Business Day.

                  All percentages resulting from any calculations on the
         Subordinated Notes will be rounded, if necessary, to the nearest one
         hundred-thousandth of a percentage point, with five one-millionths of a
         percentage point rounded upward (e.g., 9.876545% (or .09876545) being
         rounded to 9.87655% (or .0987655)), and all dollar amounts used in or
         resulting from such calculation will be rounded to the nearest cent
         (with one-half cent being rounded upward).

                  (c) Interest shall be paid to the Person in whose name such
         Subordinated Note or any predecessor Subordinated Note is registered on
         the books and records of the Company at the close of business on the
         Regular Record Date for such interest installment, which shall be
         fifteen (15) days prior to a Pre-Remarketing Interest Payment Date (the
         "PRE-REMARKETING REGULAR RECORD DATE").

                  (d) From and including the Remarketing Settlement Date on
         which Replacement Notes are issued, interest on the Replacement Notes
         will be payable quarterly in arrears (A) on February 12, May 12, August
         12 and November 12 of each year, commencing on the first such date
         following such Remarketing Settlement Date and (B) on the Maturity Date
         (each, an "INTEREST PAYMENT DATE"), and will accrue from the most
         recent date to which interest has been paid or, if no interest has been
         paid, from and including such Remarketing Settlement Date, to but
         excluding the


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         related Interest Payment Date, except as otherwise described below. The
         amount of interest payable for any quarterly period shall be computed
         on the basis of a 360-day year of twelve 30-day months. Except as
         provided in the last sentence of this paragraph, the amount of interest
         payable for any period shorter than a full quarterly period for which
         interest is computed will be computed on the basis of the actual number
         of days elapsed per 30-day month. If an Interest Payment Date is not a
         Business Day, then such Interest Payment Date will be postponed to the
         next succeeding Business Day (and without any interest or other payment
         in respect of any such delay); provided, that if such Business Day is
         in the next succeeding calendar month, such Interest Payment Date will
         be the immediately preceding Business Day.

                  (e) Interest shall be paid to the Person in whose name the
         Subordinated Note or any predecessor Subordinated Note is registered on
         the books and records of the Company, at the close of business on the
         Regular Record Date for such interest installment, which, in respect of
         (i) Subordinated Notes of which the Property Trustee is the Holder or
         (ii) a Global Subordinated Note, shall be the close of business on the
         Business Day next preceding that Interest Payment Date (the "REGULAR
         RECORD DATE"). If the Subordinated Notes are not held by the Property
         Trustee and are not represented by a Global Subordinated Note, the
         Regular Record Date for such interest installment shall be fifteen (15)
         days prior to that Interest Payment Date.

                  (f) If, at any time while the Property Trustee is the Holder
         of any Subordinated Notes, the Trust or the Property Trustee is
         required to pay any taxes, duties, assessments or governmental charges
         of whatever nature (other than withholding taxes) imposed by the United
         States, or any other taxing authority, then, in any such case, the
         Company will pay as additional interest ("ADDITIONAL INTEREST") on the
         Subordinated Notes held by the Property Trustee, such additional
         amounts as shall be required so that the net amounts received and
         retained by the Trust and the Property Trustee after paying such taxes,
         duties, assessments or other governmental charges will be equal to the
         amounts the Trust and the Property Trustee would have received had no
         such taxes, duties, assessments or other government charges been
         imposed."


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                                    ARTICLE 3


         SECTION 3.01.  Remarketing.  (a) Article 10 of the First Supplemental
Indenture is hereby amended to read in its entirety as follows:



                                   "ARTICLE 10
                             REMARKETING; RESET RATE

                  SECTION 10.01. Effectiveness of this Article; Incorporation of
         Remarketing Agreement. (a) Sections 10.02 and 10.04 shall become
         effective if and only if the Subordinated Notes have been distributed
         to the holders of the Trust Securities prior to Remarketing.
         Notwithstanding the foregoing, on the Remarketing Settlement Date
         (except in the case of a Failed Remarketing), the certificates
         representing the Subordinated Notes held by the Property Trustee shall
         be exchanged for certificates representing the Replacement Notes.

                  (b) Every Person, by virtue of having become a Holder in
         accordance with the terms of this Agreement, shall be deemed to have
         expressly assented and agreed to the terms of, and shall be bound by,
         this First Supplemental Indenture, including the terms of Exhibit B.
         Exhibit B is hereby incorporated in and expressly made a part of this
         First Supplemental Indenture.

                  SECTION 10.02.  Remarketing Procedure.

                  (a) (i) Subject to Section 10.04, the holders of a majority in
         principal amount of the Subordinated Notes (the "REQUESTING HOLDERS")
         have the right to require Remarketing of the Subordinated Notes at any
         time. The Requesting Holders may exercise this right by delivering a
         written notice to the Remarketing Agent at any time requesting a
         Remarketing of the Subordinated Notes. Upon the receipt of such notice,
         the Remarketing Agent shall immediately deliver a written notice to the
         Company on behalf of the Requesting Holders (the "REMARKETING NOTICE").
         If the Requesting Holders exercise their right to require the
         Remarketing of the Subordinated Notes, the Reset Date shall be the
         sixth Business Day (the "EXPECTED RESET DATE") after the date on which
         the Remarketing Notice is received by the Company.

                           (ii) Notwithstanding Section 10.02(a)(i):


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                                    (A) the Company may, by notice to the
                           Remarketing Agent, direct that the Reset Date be
                           delayed if the Company believes it will be unable to
                           meet the conditions to Remarketing in the absence of
                           such a delay; and

                                    (B) the Remarketing Agent may, by notice to
                           the Company, direct that the Reset Date be delayed if
                           the Remarketing Agent believes that a Remarketing
                           will not be successful in the absence of such a
                           delay;

         provided that the Company and the Remarketing Agent, in either such
         event, will use their reasonable best efforts to establish a delayed
         Reset Date that is within five Business Days after the Expected Reset
         Date, but in no event later than the 15th Business Day following the
         date on which the related Remarketing Notice was received, or the 20th
         Business Day in the case of a Renewed Remarketing (as hereinafter
         defined) to which the provisions of Section 10.04 apply (as applicable,
         the "FINAL RESET DATE").

                           (iii) If the Company and the Remarketing Agent have
                  not agreed, on or prior to the sixth Business Day preceding
                  the Final Reset Date, to a Reset Date that is not later than
                  the Final Reset Date, a Failed Remarketing shall be deemed to
                  have occurred.

                           (iv) Notwithstanding the provisions of this Article
                  10, upon receipt of a Remarketing Notice the Company shall
                  have the right, in its sole discretion, to elect to pay the
                  outstanding principal of and accrued and unpaid interest on
                  the Subordinated Notes, rather than proceed with the
                  Remarketing. The Company shall make such election by sending
                  written notice, within five Business Days after the receipt of
                  the Remarketing Notice, to the Remarketing Agent and the
                  Trustee. If the Company makes such election, it shall pay the
                  outstanding principal of and accrued and unpaid interest on
                  the Subordinated Notes to the Holders thereof on the date
                  eight Business Days after receipt of the Remarketing Notice.

                  (b) The Company shall, by notice to the Remarketing Agent no
         later than five Business Days prior to the Reset Date, select and
         specify three Reference Corporate Dealers. By 3:00 p.m., New York City
         time, on the Reset Date, the Remarketing Agent shall request Bids from
         such Reference Corporate Dealers. The Remarketing Agent or an Affiliate
         or Associated Person thereof (any such person, an "AFFILIATED BIDDER")
         may, at its option, enter a Bid. The Remarketing Agent shall disclose
         to the


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         Company the Bids obtained and determine the lowest Bid Rate from among
         the Bids obtained on the Reset Date (the "WINNING BID RATE"). By
         approximately 4:30 p.m., New York City time, on the Reset Date, the
         Remarketing Agent shall notify the Company and the Trustee of the
         Winning Bid Rate. If on a Reset Date, Bids are not submitted by at
         least two Reference Corporate Dealers, or if the lowest Bid submitted
         would result in a Winning Bid Rate in excess of the rate permitted by
         applicable law, the Remarketing shall be deemed to be a Failed
         Remarketing on the corresponding Remarketing Settlement Date. The
         Winning Bid Rate determined by the Remarketing Agent, absent manifest
         error, shall be binding and conclusive upon the holders of the
         Subordinated Notes, the Company and the Trust.

                  (c) On the Reset Date, the Remarketing Agent shall designate
         as the Secondary Purchaser (the "SECONDARY PURCHASER") the Reference
         Corporate Dealer providing the Bid containing the Winning Bid Rate. If
         the Winning Bid Rate is specified in the Bids submitted by two or more
         bidders, the Remarketing Agent shall, in consultation with the Company,
         designate one of such bidders as the Secondary Purchaser.

                  (d) On the Reset Date, the Secondary Purchaser shall enter
         into a Secondary Purchase Agreement for the purchase by such Secondary
         Purchaser at the Remarketing Price of the aggregate principal amount of
         Subordinated Notes, with an Interest Rate equal to the Winning Bid Rate
         and with a Maturity Date on the Remarketed Redemption Date.

                  (e) If a Remarketing has occurred pursuant to this Section
         10.02 but settlement of the purchase and sale of the Subordinated Notes
         does not occur on the corresponding Remarketing Settlement Date, then,
         unless the provisions of Section 10.04 with respect to a Renewed
         Remarketing shall apply, a Failed Remarketing shall be deemed to have
         occurred on such Remarketing Settlement Date.

                  (f) At the time and in the manner specified in the Secondary
         Purchase Agreement, the Secondary Purchaser shall pay on the
         Remarketing Settlement Date to the Remarketing Agent on behalf of the
         Holders of the Subordinated Notes an amount of cash equal to the
         Remarketing Price.

                  (g) Unless otherwise agreed among the Remarketing Agent, the
         Paying Agent and any Former Holder, the Remarketing Agent shall
         promptly pay the Remarketing Price to the Paying Agent, acting solely
         as agent for the Former Holders, and the Paying Agent shall pay such
         amount to the Former Holders in the manner specified in Section 2.02 of
         the Base


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         Indenture for payments of interest and as is otherwise specified
         herein, except that the Record Date therefor shall be the Business Day
         immediately preceding the Remarketing Settlement Date.

                  (h) The obligation of the Remarketing Agent to make payment to
         the Former Holders in connection with the Remarketing shall be limited
         to the extent that the Secondary Purchaser has delivered the
         Remarketing Price therefor to the Remarketing Agent.

                  (i) Any outstanding Subordinated Notes purchased on the
         Remarketing Settlement Date shall be deemed to be transferred to the
         Secondary Purchaser and shall be replaced in the manner provided in
         Section 10.02(j). On and after the Remarketing Settlement Date (except
         in the event of (y) a Failed Remarketing or (z) a failure by the
         Company to pay on the Remarketing Settlement Date all accrued interest
         on the Subordinated Notes to such Remarketing Settlement Date), (A) the
         Company shall make no further payments to, and the Company shall have
         no further obligations under this First Supplemental Indenture (or the
         Indenture) in respect of, the holders of such replaced Subordinated
         Notes (the "FORMER HOLDERS"), (B) the Company shall only be obligated
         to make payments to the holders of Replacement Notes and (C) the
         Subordinated Notes of the Former Holders shall no longer represent an
         obligation of the Company, but shall only represent a right to receive
         the proceeds of the Remarketing from the Paying Agent.

                  (j) The Company shall cause replacement certificates
         evidencing the remarketed Subordinated Notes (or, if the Preferred
         Securities have been remarketed, appropriately revised Subordinated
         Notes) to be executed by the Company and authenticated by the Trustee
         in accordance with the provisions of Section 2.03 of the Base Indenture
         (the "REPLACEMENT NOTES"). If the Subordinated Notes were Remarketed,
         the Replacement Notes shall be delivered to the purchaser or purchasers
         of the remarketed Subordinated Notes in accordance with the terms of
         the Secondary Purchase Agreement. If the Preferred Securities were
         Remarketed, the Replacement Notes shall be delivered to the Property
         Trustee of the Trust.

                  SECTION 10.03. Reset of Interest Rate and Maturity Date. From
         and including the Remarketing Settlement Date on which Replacement
         Notes are issued, if the Subordinated Notes are remarketed pursuant to
         Article 10 or the Preferred Securities are remarketed pursuant to
         Article 6 of the Trust Agreement, the Interest Rate on the Subordinated
         Notes shall be the Winning Bid Rate and the Maturity Date shall be the
         Remarketed Maturity Date.


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<PAGE>   13
                  SECTION 10.04. Renewed Remarketing. If a Remarketing has
         occurred pursuant to Section 10.02 that would be a Failed Remarketing
         pursuant to Section 10.02(e), because the purchase and sale of the
         Subordinated Notes do not take place on the corresponding Remarketing
         Settlement Date, and the reason for such failure shall, in the good
         faith determination of the Remarketing Agent, result from facts or
         circumstances that are not due to the action or inaction of the
         Company, then the provisions of Section 10.02 shall apply to a second
         remarketing (a "RENEWED REMARKETING") of the Subordinated Notes, except
         that the Expected Reset Date shall be the sixth Business Day following
         such corresponding Remarketing Settlement Date; provided that upon the
         occurrence of a Failed Remarketing pursuant to Section 10.02, only one
         Renewed Remarketing may occur pursuant to this Section 10.04, and no
         Renewed Remarketing shall occur after the Final Reset Date.

                  SECTION 10.05. Failed Remarketing. The Remarketing Agent shall
         give notice of any Failed Remarketing on the date such Failed
         Remarketing occurs, or is deemed to occur, by 4:00 p.m., New York City
         time, on the date of such Failed Remarketing, to the Company, the
         Trustee and the Paying Agent."



                                    ARTICLE 4


         SECTION 4.01. Ratification of Base Indenture, the First Supplemental
Indenture, the Second Supplemental Indenture, the Third Supplemental Indenture
and the Fifth Supplemental Indenture: Sixth Supplemental Indenture Controls. The
Base Indenture, as supplemented by the First Supplemental Indenture, the Second
Supplemental Indenture, the Third Supplemental Indenture, the Fifth Supplemental
Indenture and this Sixth Supplemental Indenture, is in all respects ratified and
confirmed, and this Sixth Supplemental Indenture shall be deemed part of the
Base Indenture in the manner and to the extent herein and therein provided. The
provisions of this Sixth Supplemental Indenture shall supersede the provisions
of the Base Indenture, the First Supplemental Indenture, the Second Supplemental
Indenture, the Third Supplemental Indenture and the Fifth Supplemental Indenture
to the extent the Base Indenture, the First Supplemental Indenture, the Second
Supplemental Indenture, the Third Supplemental Indenture or the Fifth
Supplemental Indenture is inconsistent herewith.

         SECTION 4.02. Trustee Not Responsible for Recitals. The recitals herein
contained are made by the Company and not by the Trustee, and the Trustee
assumes no responsibility for the correctness thereof. The Trustee makes no


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<PAGE>   14
representation as to the validity or sufficiency of this Sixth Supplemental
Indenture.

         SECTION 4.03. Governing Law. This Sixth Supplemental Indenture shall be
governed by and construed in accordance with the laws of the State of New York,
as applied to contracts made and performed within the State of New York, without
regard to its principles of conflicts of laws.

         SECTION 4.04. Severability. If any provision in the Base Indenture, the
First Supplemental Indenture, the Second Supplemental Indenture, the Third
Supplemental Indenture, the Fifth Supplemental Indenture, this Sixth
Supplemental Indenture or in the Subordinated Notes shall be invalid, illegal or
unenforceable, the validity, legality and enforceability of the remaining
provisions shall not in any way be affected or impaired thereby.

         SECTION 4.05. Counterparts. The parties may sign any number of copies
of this Sixth Supplemental Indenture. Each signed copy shall be an original, but
all of them together represent the same agreement. Any signed copy shall be
sufficient proof of this Sixth Supplemental Indenture.

         SECTION 4.06.  Terms Defined.  All terms defined elsewhere in the
Indenture shall have the same meanings when used herein.


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<PAGE>   15
         IN WITNESS WHEREOF, the parties hereto have caused this Sixth
Supplemental Indenture to be duly executed as of the day and year first above
written.


                                                  HERCULES INCORPORATED,
                                                           as Issuer


                                                  By: __________________________
                                                       Name:
                                                       Title:



                                                  THE CHASE MANHATTAN BANK,
                                                         as Trustee


                                                  By: __________________________
                                                       Name:
                                                       Title:
<PAGE>   16
                                                                       EXHIBIT A

                            FORM OF SUBORDINATED NOTE

                                SUBORDINATED NOTE

THE SECURITY EVIDENCED HEREBY HAS NOT BEEN REGISTERED UNDER THE U.S. SECURITIES
ACT OF 1933, AS AMENDED (THE "SECURITIES ACT"), AND, ACCORDINGLY, MAY NOT BE
OFFERED OR SOLD EXCEPT AS SET FORTH IN THE FOLLOWING SENTENCE. BY ITS
ACQUISITION HEREOF, THE HOLDER (1) REPRESENTS THAT IT IS A "QUALIFIED
INSTITUTIONAL BUYER" (AS DEFINED IN RULE 144A UNDER THE SECURITIES ACT), (2)
AGREES THAT IT WILL NOT PRIOR TO THE EXPIRATION OF THE HOLDING PERIOD APPLICABLE
TO SALES OF THE SECURITY EVIDENCED HEREBY UNDER RULE 144(K) UNDER THE SECURITIES
ACT (OR ANY SUCCESSOR PROVISION) RESELL OR OTHERWISE TRANSFER THE SECURITY
EVIDENCED HEREBY EXCEPT (A) TO HERCULES INCORPORATED OR ANY SUBSIDIARY THEREOF,
(B) TO A QUALIFIED INSTITUTIONAL BUYER IN COMPLIANCE WITH RULE 144A UNDER THE
SECURITIES ACT OR (C) TO A SECONDARY PURCHASER (AS DEFINED IN THE AMENDED AND
RESTATED TRUST AGREEMENT OF HERCULES TRUST V DATED AS OF NOVEMBER 12, 1998 (AS
AMENDED FROM TIME TO TIME, THE "TRUST AGREEMENT")) THAT HAS ENTERED INTO A
SECONDARY PURCHASE AGREEMENT (AS DEFINED IN THE TRUST AGREEMENT) WITH THE TRUST,
(3) AGREES THAT IT WILL DELIVER TO EACH PERSON TO WHOM THE SECURITY EVIDENCED
HEREBY IS TRANSFERRED A NOTICE SUBSTANTIALLY TO THE EFFECT OF THIS LEGEND AND
(4) AGREES WITH RESPECT TO ANY TRANSFER OCCURRING PRIOR TO THE REMARKETING DATE
ON WHICH REPLACEMENT NOTES ARE ISSUED TO ANY PERSON OTHER THAN THE PROPERTY
TRUSTEE, TO PROVIDE TO THE INDENTURE TRUSTEE A DULY EXECUTED CERTIFICATE
SUBSTANTIALLY TO THE EFFECT OF CLAUSES (1), (2) AND (3), ABOVE. AT THE REQUEST
OF THE HOLDER, THIS LEGEND WILL BE REMOVED AFTER THE EXPIRATION OF THE HOLDING
PERIOD APPLICABLE TO THE SALE OF THE SECURITY EVIDENCED HEREBY UNDER RULE 144(K)
UNDER THE SECURITIES ACT.

FROM AND AFTER THE REMARKETING SETTLEMENT DATE ON WHICH REPLACEMENT NOTES ARE
ISSUED TO ANY PERSON OTHER THAN THE PROPERTY TRUSTEE, THIS CERTIFICATE SHALL
REPRESENT ONLY THE RIGHT TO RECEIVE THE REMARKETING PRICE, AS PROVIDED IN THE
TRUST AGREEMENT, AND SHALL NO LONGER REPRESENT AN OBLIGATION OF THE COMPANY.


                                       A-1
<PAGE>   17
THIS SECURITY WAS ISSUED WITH ORIGINAL ISSUE DISCOUNT FOR UNITED STATES FEDERAL
INCOME TAX PURPOSES. THE ISSUE PRICE, AMOUNT OF ORIGINAL ISSUE DISCOUNT, ISSUE
DATE AND YIELD TO MATURITY WILL BE PROMPTLY MADE AVAILABLE UPON REQUEST TO THE
VICE PRESIDENT - TAXES (AT (302) 594-5887) OR THE SENIOR VICE PRESIDENT AND
CHIEF FINANCIAL OFFICER (AT (302) 594-5175), HERCULES INCORPORATED, HERCULES
PLAZA, 1313 NORTH MARKET STREET, WILMINGTON, DE 19894-0001.


                                       A-2
<PAGE>   18
No. SN-03                                                     CUSIP NO. ________



                              HERCULES INCORPORATED

              AUCTION RATE RESET JUNIOR SUBORDINATED NOTE SERIES A

                 Hercules Incorporated, a Delaware corporation (the "COMPANY",
which term includes any successor corporation under the Indenture hereinafter
referred to), for value received, hereby promises to pay to The Chase Manhattan
Bank, as Property Trustee, for Hercules Trust V, or registered assigns, the
principal sum of Two Hundred Six Million Two Hundred Thousand Dollars
($206,200,000) on February 9, 2002, or such other date as may be provided
pursuant to the terms of the Indenture.

         (a) Interest on the principal amount of each Subordinated Note will
accrue and be payable at a rate (the "INTEREST RATE") per annum equal to:

                           (i) from and including November 12, 1998 to but
                  excluding February 9, 2000, LIBOR plus 175 basis points;

                           (ii) from and including February 9, 2000 to but
                  excluding the earlier of (A) the Remarketing Settlement Date
                  on which Replacement Notes are issued and (B) the date such
                  principal amount is paid, LIBOR plus 150 basis points;

                           (iii) from and including the Remarketing Settlement
                  Date on which Replacement Notes are issued to but excluding
                  the date such principal amount is paid, the Winning Bid Rate;
                  and

                           (iv) notwithstanding clauses (i), (ii) and (iii)
                  above, if the Company fails to pay the principal amount on the
                  date such amount becomes due, then from and including such due
                  date to but excluding the date such principal amount is paid,
                  the applicable Interest Rate in effect on the due date,
                  compounded quarterly, but only to the extent permitted by
                  applicable law.

                 Interest that is not paid when due will bear additional
interest thereon compounded quarterly at the applicable Interest Rate in effect
on the due date specified above (to the extent permitted by applicable law). The
term "INTEREST", as used herein, includes any such additional interest unless
otherwise stated.


                                       A-3
<PAGE>   19
         (b) Until the Remarketing Settlement Date on which Replacement Notes
are issued, interest on the Subordinated Notes will be payable quarterly in
arrears (A) on February 12, May 12, August 12 and November 12 of each year,
commencing February 12, 1999 and (B) on such Remarketing Settlement Date (each,
a "PRE-REMARKETING INTEREST PAYMENT DATE"), and will accrue from and including
the most recent date to which interest has been paid or, if no interest has been
paid, from November 12, 1998, to but excluding the related Pre-Remarketing
Interest Payment Date, except as otherwise described below.

                 The Interest Rate in effect for the period from and including
November 12, 1998 to but excluding February 12, 1999 shall be the rate
determined by the Calculation Agent two London Banking Days prior to November
12, 1998 and shall equal LIBOR plus 175 basis points. The Interest Rate in
effect from and including February 12, 1999 to but excluding February 9, 2000,
for each quarterly period from and including the immediately preceding
Pre-Remarketing Interest Payment Date to but excluding the applicable
Pre-Remarketing Interest Payment Date, shall be determined by the Calculation
Agent two London Banking Days prior to such immediately preceding
Pre-Remarketing Interest Payment Date (a "DATE OF DETERMINATION") and shall
equal LIBOR plus 175 basis points. The Interest Rate in effect thereafter, for
each quarterly period from and including the immediately preceding
Pre-Remarketing Interest Payment Date to but excluding the applicable
Pre-Remarketing Interest Payment Date, shall be determined by the Calculation
Agent two London Banking Days prior to such immediately preceding
Pre-Remarketing Interest Payment Date and shall equal LIBOR plus 150 basis
points. The amount of interest payable on February 12, 2000 shall reflect the
pro rata application of the two Interest Rates applicable to the calculation
period for such interest payment for each day on which the applicable Interest
Rate was in effect. The amount of interest payable for any quarterly period
shall be computed on the basis of a 360-day year of twelve 30-day months. Except
as provided in the last sentence of this paragraph, the amount of interest
payable for any period shorter than a full quarterly period for which interest
is computed will be computed on the basis of the actual number of days elapsed
per 30-day month. If a Pre-Remarketing Interest Payment Date is not a Business
Day, then such Pre-Remarketing Interest Payment Date will be the next succeeding
Business Day, except if such Business Day is in the next succeeding calendar
month, such Pre-Remarketing Distribution Date will be the immediately preceding
Business Day.

       All percentages resulting from any calculations on the Subordinated Notes
will be rounded, if necessary, to the nearest one hundred-thousandth of a
percentage point, with five one-millionths of a percentage point rounded upward
(e.g., 9.876545% (or .09876545) being rounded to 9.87655% (or .0987655)), and
all dollar amounts used in or resulting from such calculation will be rounded to
the nearest cent (with one-half cent being rounded upward).


                                       A-4
<PAGE>   20
(c) Interest shall be paid to the Person in whose name such Subordinated Note or
any predecessor Subordinated Note is registered on the books and records of the
Company at the close of business on the Regular Record Date for such interest
installment, which shall be fifteen (15) days prior to a Pre-Remarketing
Interest Payment Date (the "PRE-REMARKETING REGULAR RECORD DATE").

(d) From and including the Remarketing Settlement Date on which Replacement
Notes are issued, interest on the Replacement Notes will be payable quarterly in
arrears (A) on February 12, May 12, August 12 and November 12 of each year,
commencing on the first such date following such Remarketing Settlement Date and
(B) on the Maturity Date (each, an "INTEREST PAYMENT DATE"), and will accrue
from the most recent date to which interest has been paid or, if no interest has
been paid, from and including such Remarketing Settlement Date, to but excluding
the related Interest Payment Date, except as otherwise described below. The
amount of interest payable for any quarterly period shall be computed on the
basis of a 360-day year of twelve 30-day months. Except as provided in the last
sentence of this paragraph, the amount of interest payable for any period
shorter than a full quarterly period for which interest is computed will be
computed on the basis of the actual number of days elapsed per 30-day month. If
an Interest Payment Date is not a Business Day, then such Interest Payment Date
will be postponed to the next succeeding Business Day (and without any interest
or other payment in respect of any such delay); provided, that if such Business
Day is in the next succeeding calendar month, such Interest Payment Date will be
the immediately preceding Business Day.

(e) Interest shall be paid to the Person in whose name the Subordinated Note or
any predecessor Subordinated Note is registered on the books and records of the
Company, at the close of business on the Regular Record Date for such interest
installment, which, in respect of (i) Subordinated Notes of which the Property
Trustee is the Holder or (ii) a Global Subordinated Note, shall be the close of
business on the Business Day next preceding that Interest Payment Date (the
"REGULAR RECORD DATE"). If the Subordinated Notes are not held by the Property
Trustee and are not represented by a Global Subordinated Note, the Regular
Record Date for such interest installment shall be fifteen (15) days prior to
that Interest Payment Date.

The indebtedness evidenced by this Subordinated Note is, to the extent provided
in the Indenture, subordinate and junior in right of payment to the prior
payment in full of all existing and future Senior Indebtedness, and this
Subordinated Note is issued subject to the provisions of the Indenture with
respect thereto. Each Holder of this Subordinated Note, by accepting the same,
(a) agrees to and shall be bound by such provisions, (b) authorizes and directs
the Trustee on his or her behalf to take such action as may be necessary or
appropriate to acknowledge or effectuate the subordination so provided and (c)
appoints the Trustee his or her

                                      A-5
<PAGE>   21
attorney-in-fact for any and all such purposes. Each Holder hereof, by his or
her acceptance hereof, hereby waives all notice of the acceptance of the
subordination provisions contained herein and in the Indenture by each holder of
Senior Indebtedness, whether now outstanding or hereafter incurred, and waives
reliance by each such holder upon said provisions.

This Subordinated Note shall not be entitled to any benefit under the Indenture
hereinafter referred to, be valid or become obligatory for any purpose until the
Certificate of Authentication hereon shall have been signed by or on behalf of
the Trustee.

The provisions of this Subordinated Note are continued on the reverse side
hereof and such continued provisions shall for all purposes have the same effect
as though fully set forth at this place.


                                       A-6
<PAGE>   22
       IN WITNESS WHEREOF, the Company has caused this instrument to be
executed.

                                        HERCULES INCORPORATED

                                        By: ____________________________________
                                             Name:
                                             Title

Attest:


By: ______________________________
     Name:
     Title:
<PAGE>   23
                          CERTIFICATE OF AUTHENTICATION


This is one of the Subordinated Notes of the series of Debentures described in
the within-mentioned Indenture.

Dated:

THE CHASE MANHATTAN BANK,
as Trustee                                  or as Authentication Agent



By _______________________________          By _________________________________
    Authorized Signatory                        Authorized Signatory
<PAGE>   24
                                [REVERSE OF NOTE]

         This Subordinated Note is one of a duly authorized series of Debentures
of the Company (herein sometimes referred to as the "DEBENTURES"), specified in
the Indenture, all issued or to be issued in one or more series under and
pursuant to a Junior Subordinated Debenture Indenture dated as of November 12,
1998, duly executed and delivered between the Company and The Chase Manhattan
Bank, as Trustee (the "TRUSTEE"), as supplemented by a First Supplemental
Indenture dated as of November 12, 1998, a Second Supplemental Indenture dated
as of July 6, 1999, a Third Supplemental Indenture dated as of October 25, 1999,
a Fifth Supplemental Indenture dated as of January 24, 2000 and a Sixth
Supplemental Indenture dated as of February 9, 2000 (the Indenture as so
supplemented, the "INDENTURE"), to which Indenture and all indentures
supplemental thereto reference is hereby made for a description of the rights,
limitations of rights, obligations, duties and immunities thereunder of the
Trustee, the Company and the Holders of the Subordinated Notes. By the terms of
the Indenture, the Debentures are issuable thereunder in series that may vary as
to amount, date of maturity, rate of interest and in other respects as provided
in the Indenture. This series of Debentures is limited in aggregate principal
amount as specified in said Sixth Supplemental Indenture and herein sometimes
referred to as the "SUBORDINATED NOTES."

         Because of the occurrence and continuation of a Special Event or a
Failed Remarketing, in certain circumstances, this Subordinated Note may become
due and payable at the principal amount together with any interest accrued
thereon (the "REDEMPTION PRICE"). The Redemption Price shall be paid prior to
12:00 noon, New York City time, on the date of such redemption or at such
earlier time as the Company determines.

         In case an Event of Default, as defined in the Indenture, shall have
occurred and be continuing, the principal of all of the Subordinated Notes may
be declared, and upon such declaration shall become, due and payable, in the
manner, with the effect and subject to the conditions provided in the Indenture.

         The Indenture contains provisions permitting the Company and the
Trustee, with the consent of the Holders of at least a majority in aggregate
principal amount of the Debentures of each series affected thereby then
outstanding (and, in the case of any series of Debentures held as assets of a
Trust and with respect to which a Dissolution Event has not theretofore
occurred, such consent of holders of the Preferred Securities and the Common
Securities of such Trust as may be required under the Trust Agreement), as
defined in the Indenture, to reduce the principal amount of such Debentures;
reduce the percentage of the principal amount of such Debentures the Holders of
which must consent to an amendment of this Indenture or a waiver; change (i) the
stated maturity of the


                                      A-9
<PAGE>   25
principal of or the interest on such Debentures, or (ii) the rate of interest
(or the manner of calculation thereof) on such Debentures, change adversely to
the Holders the redemption, conversion or exchange provisions applicable to such
Debentures, if any; change the currency in respect of which the payments on such
Debentures are to be made; make any change in the Subordination provisions of
the Indenture (Article 10) that adversely affects the rights of the Holders of
the Debentures or any change to any other Section hereof that adversely affects
their rights; or change the direct action rights of holders of Preferred
Securities; provided that, in the case of the outstanding Debentures of a series
then held by a Trust, no such amendment shall be made that adversely affects the
holders of the Preferred Securities of that Trust, and no waiver of any Event of
Default with respect to the Debentures of that series or compliance with any
covenant under this Indenture shall be effective, without the prior consent of
the holders of at least a majority of the aggregate liquidation amount of the
outstanding Preferred Securities of that Trust or the holder of each such
Preferred Security, as applicable.

         A supplemental indenture that changes or eliminates any covenant or
other provision of this Indenture that has expressly been included solely for
the benefit of one or more particular series of Debentures, or which modifies
the rights of the Holders of Debentures of such series with respect to such
covenant or other provision, shall be deemed not to affect the rights under this
Indenture of the Holders of Debentures of any other series.

         No reference herein to the Indenture and no provision of this
Subordinated Note or of the Indenture shall alter or impair the obligation of
the Company, which is absolute and unconditional, to pay the principal of and
premium, if any, and interest on this Subordinated Note at the time and place
and at the rate or rates and in the currency herein prescribed.

         As provided in the Indenture and subject to certain limitations herein
and therein set forth, this Subordinated Note is transferable by the registered
Holder hereof on the Register of the Company, upon surrender of this
Subordinated Note for registration of transfer at the office or agency of the
Trustee in the City and State of New York accompanied by a written instrument or
instruments of transfer in form satisfactory to the Company or the Trustee duly
executed by the registered Holder hereof or his attorney duly authorized in
writing, and thereupon one or more new Subordinated Notes of authorized
denominations and for the same aggregate principal amount and series will be
issued to the designated transferee or transferees. No service charge will be
made for any such transfer, but the Company may require payment of a sum
sufficient to cover any tax or other governmental charge payable in relation
thereto.


                                      A-10
<PAGE>   26
         Prior to due presentment for registration of transfer of this
Subordinated Note, the Company, the Trustee, any paying agent and the Registrar
may deem and treat the registered holder hereof as the absolute owner hereof
(whether or not this Subordinated Note shall be overdue and notwithstanding any
notice of ownership or writing hereon made by anyone other than the Registrar)
for the purpose of receiving payment of or on account of the principal hereof
and premium, if any, and interest due hereon and for all other purposes, and
neither the Company nor the Trustee nor any paying agent nor any Registrar shall
be affected by any notice to the contrary.

         No recourse shall be had for the payment of the principal of or the
interest on this Subordinated Note, or for any claim based hereon, or otherwise
in respect hereof, or based on or in respect of the Indenture, against any
incorporator, stockholder, officer or director, past, present or future, as
such, of the Company or of any predecessor or successor corporation, whether by
virtue of any constitution, statute or rule of law, or by the enforcement of any
assessment or penalty or otherwise, all such liability being, by the acceptance
hereof and as part of the consideration for the issuance hereof, expressly
waived and released.

         The Subordinated Notes of this series are issuable only in registered
form without coupons in denominations of $1,000 and any integral multiple of
$1,000 thereof. The Subordinated Notes may be transferred or exchanged only in
minimum denominations of $100,000 and integral multiples of $1,000 in excess
thereof, and any attempted transfer, sale or other disposition of Subordinated
Notes in a denomination of less than $100,000 shall be deemed void and of no
legal effect whatsoever.

         All terms used in this Subordinated Note that are defined in the
Indenture shall have the meanings assigned to them in the Indenture.

         THE INDENTURE AND THE DEBENTURES SHALL BE GOVERNED BY AND CONSTRUED IN
ACCORDANCE WITH THE LAWS OF THE STATE OF NEW YORK WITHOUT REGARD TO ITS
PRINCIPLES OF CONFLICTS OF LAWS.


                                      A-11