<PAGE>
 
                                                                    EXHIBIT 4.6 
--------------------------------------------------------------------------------

                          RADNOR HOLDINGS CORPORATION
                                  as Issuer,

                             WINCUP HOLDINGS, INC.
                          RADNOR CHEMICAL CORPORATION
                             RADNOR DELAWARE, INC.
                            RADNOR MANAGEMENT, INC.
                             STYROCHEM U.S., LTD.
                              WINCUP TEXAS, LTD.
                             STYROCHEM GP, L.L.C.
                             STYROCHEM LP, L.L.C.
                               WINCUP GP, L.L.C.
                                      and
                               WINCUP LP, L.L.C.

                                 as Guarantors

                                      and

                           FIRST UNION NATIONAL BANK
                                  as Trustee

                             _____________________

                         FIFTH SUPPLEMENTAL INDENTURE

                         Dated as of January 21, 1999



        (Supplementing a Trust Indenture dated as of December 5, 1996,
as amended by a First Supplemental Indenture dated as of December 17, 1996, and
as amended by a Second Supplemental Indenture dated as of October 15, 1997, and
as amended by a Third Supplemental Indenture dated as of February 9, 1998, and
    as amended by a Fourth Supplemental Indenture dated as of July 8, 1998)

                                 $100,000,000
                           10% Senior Notes due 2003

--------------------------------------------------------------------------------
                                        
<PAGE>
 
     THIS FIFTH SUPPLEMENTAL INDENTURE, dated as of the 21st day of January,
1999 (this "Fifth Supplemental Indenture"), is among RADNOR HOLDINGS
CORPORATION, a Delaware corporation (the "Company"), WINCUP HOLDINGS, INC., a
Delaware corporation, RADNOR CHEMICAL CORPORATION (formerly known as SP
Acquisition Co.), a Delaware corporation, RADNOR DELAWARE, INC., a Delaware
corporation, RADNOR MANAGEMENT, INC., a Delaware corporation, STYROCHEM U.S.,
LTD. (formerly known as StyroChem International, Inc. and StyroChem U.S., Inc.),
a Texas limited partnership (collectively, the "Guarantors"), WINCUP TEXAS,
LTD., a Texas limited partnership, STYROCHEM GP, L.L.C., a Delaware limited
liability company, STYROCHEM LP, L.L.C., a Delaware limited liability company,
WINCUP GP, L.L.C., a Delaware limited liability company and WINCUP LP, L.L.C., a
Delaware limited liability company (collectively the "New Guarantors") and FIRST
UNION NATIONAL BANK, as trustee (the "Trustee").

                                   RECITALS:

     The Company, the Guarantors and the Trustee are parties to a certain
Indenture dated December 5, 1996, as amended by a First Supplemental Indenture
dated as of December 17, 1996, as amended by a Second Supplemental Indenture
dated as of October 15, 1997, as amended by a Third Supplemental Indenture dated
as of February 9, 1998, as amended by a Fourth Supplemental Indenture dated as
of July 8, 1998 (as amended, the "Indenture"), relating to the creation by the
Company of an issue of $100,000,000 of its 10% Senior Notes, due 2003 (the
"Securities");

     Each Guarantor has issued a guarantee of the Securities (collectively, the
"Guarantees") pursuant to which the Guarantors have guaranteed, in accordance
with Article Thirteen of the Indenture, all Indenture Obligations (as such term
is defined in the Indenture); and

     The Company, the Guarantors, the New Guarantors and the Trustee now desire
to enter into this Fifth Supplemental Indenture pursuant to Section 901(vi) of
the Indenture, without the consent of the Holders, in order to add the New
Guarantors as Guarantors and Restricted Subsidiaries under the Indenture;

     Capitalized terms used herein without definition shall have the meanings
given such terms in the Indenture.

     NOW, THEREFORE, THIS FIFTH SUPPLEMENTAL INDENTURE WITNESSETH:

     For and in consideration of the premises and for other good and valuable
consideration, it is covenanted and agreed, for the benefit of each other and
for the equal and proportionate benefit of the Holders of the Securities issued
under the Indenture, as follows:
<PAGE>
 
                                  ARTICLE ONE

                  JOINDER AND GUARANTEE OF THE NEW GUARANTORS

     Section 101. The New Guarantors hereby absolutely, unconditionally and
irrevocably guarantee, on a joint and several basis with the Guarantors, to the
Trustee and the Holders, as if each New Guarantor was the principal debtor, the
punctual payment and performance when due of all Indenture Obligations (which
for purposes of this Guarantee shall also be deemed to include all commissions,
fees, charges, costs and expenses (including reasonable legal fees and
disbursements of one counsel) arising out of or incurred by the Trustee or the
Holders in connection with the enforcement of this Guarantee).  This Guarantee
shall rank pari passu with any Senior Indebtedness of New Guarantors and shall
be subject in all respects to, and governed by all of the terms and provisions
applicable to Guarantees in, the Indenture, including without limitation Article
Thirteen thereof.

     Section 102.  As of the date hereof, all references to the "Guarantors" in
the Indenture shall be deemed to refer collectively to: (i) the Guarantors in
existence on the date hereof and (ii) New Guarantors.

     IN WITNESS WHEREOF, the parties hereto have caused this Fifth Supplemental
Indenture to be duly executed, all as of the day and year first above written.

                                        RADNOR HOLDINGS CORPORATION


Attest: /s/ Caroline J. Williamson      By: /s/ Michael T. Kennedy
        --------------------------          ---------------------------
        Caroline J. Williamson              Michael T. Kennedy
        Secretary                           President


                                        WINCUP HOLDINGS, INC.


Attest: /s/ Caroline J. Williamson      By: /s/ Michael T. Kennedy
        --------------------------          ---------------------------
        Caroline J. Williamson              Michael T. Kennedy
        Secretary                           President


                                        RADNOR CHEMICAL CORPORATION



Attest: /s/ Caroline J. Williamson      By: /s/ Michael T. Kennedy
        --------------------------          ---------------------------
        Caroline J. Williamson              Michael T. Kennedy
        Secretary                           President

                                       3
<PAGE>
 
                                        RADNOR DELAWARE, INC.



Attest: /s/ Caroline J. Williamson      By: /s/ Michael T. Kennedy
        --------------------------          ---------------------------
        Caroline J. Williamson              Michael T. Kennedy
        Secretary                           President


                                        RADNOR MANAGEMENT, INC.


Attest: /s/ Caroline J. Williamson      By: /s/ Michael T. Kennedy
        --------------------------          ---------------------------
        Caroline J. Williamson              Michael T. Kennedy
        Secretary                           President


                                        STYROCHEM U.S., LTD.
                                        By:  StyroChem GP, L.L.C.,
                                              its general partner,
                                        By:  Radnor Chemical Corporation,
                                              its sole member



Attest: /s/ Caroline J. Williamson      By: /s/ Michael T. Kennedy
        --------------------------          ---------------------------
        Caroline J. Williamson              Michael T. Kennedy
        Secretary                           President


                                        WINCUP TEXAS, LTD.
                                        By:  WinCup GP, L.L.C.,
                                              its general partner,
                                        By:  WinCup Holdings, Inc.,
                                              its sole member


Attest: /s/ Caroline J. Williamson      By: /s/ Michael T. Kennedy
        --------------------------          ---------------------------
        Caroline J. Williamson              Michael T. Kennedy
        Secretary                           President

                                       4
<PAGE>
 
                                        STYROCHEM GP, L.L.C.
                                        By: Radnor Chemical Corporation,
                                              its sole member



Attest: /s/ Caroline J. Williamson      By: /s/ Michael T. Kennedy
        --------------------------          ---------------------------
        Caroline J. Williamson              Michael T. Kennedy
        Secretary                           President



                                        STYROCHEM LP, L.L.C.
                                        By: Radnor Chemical Corporation,
                                              its sole member


Attest: /s/ Caroline J. Williamson      By: /s/ Michael T. Kennedy
        --------------------------          ---------------------------
        Caroline J. Williamson              Michael T. Kennedy
        Secretary                           President


                                        WINCUP GP, L.L.C.
                                        By: WinCup Holdings, Inc.,
                                              its sole member


Attest: /s/ Caroline J. Williamson      By: /s/ Michael T. Kennedy
        --------------------------          ---------------------------
        Caroline J. Williamson              Michael T. Kennedy
        Secretary                           President


                                        WINCUP LP, L.L.C.
                                        By: WinCup Holdings, Inc.,
                                              its sole member


Attest: /s/ Caroline J. Williamson      By: /s/ Michael T. Kennedy
        --------------------------          ---------------------------
        Caroline J. Williamson              Michael T. Kennedy
        Secretary                           President

                                        FIRST UNION NATIONAL BANK,
                                        as Trustee


Attest: /s/ Ralph E. Jones              By: /s/ Alan G. Finn
        --------------------------          ---------------------------
        Name: Ralph E. Jones                Alan G. Finn
        Title: Corporate Trust Officer      Assistant Vice President

                                       5