<PAGE>

                                                                     Exhibit 4.8

                                                                   Executed Copy
-------------------------------------------------------------------------------

                           RADNOR HOLDINGS CORPORATION
                                   as Issuer,

                              WINCUP HOLDINGS, INC.
                           RADNOR CHEMICAL CORPORATION
                             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.
                                WINCUP LP, L.L.C.
                        RADNOR MANAGEMENT DELAWARE, INC.
                            STYROCHEM DELAWARE, INC.
                          WINCUP EUROPE DELAWARE, INC.
                         STYROCHEM EUROPE DELAWARE, INC.
                                       and
                            RADNOR DELAWARE II, INC.
                                  as Guarantors

                                       and

                            FIRST UNION NATIONAL BANK
                                   as Trustee

                              ____________________

                         SEVENTH SUPPLEMENTAL INDENTURE

                          Dated as of January 29, 2002

   (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, and as
 amended by a Fifth Supplemental Indenture dated as of January 21, 1999, and as
   amended by a Sixth Supplemental Indenture dated as of March 23, 1999)

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

-------------------------------------------------------------------------------
<PAGE>

                  THIS SEVENTH SUPPLEMENTAL INDENTURE, dated as of the 29th
day of January, 2002 (this "Seventh 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 MANAGEMENT, INC., a Delaware
corporation, STYROCHEM U.S., LTD. (formerly known as StyroChem International,
Inc. and StyroChem U.S., Inc.), a Texas limited partnership 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, WINCUP LP, L.L.C., a Delaware
limited liability company, RADNOR MANAGEMENT DELAWARE, INC., a Delaware
corporation, STYROCHEM DELAWARE, INC., a Delaware corporation, and WINCUP
EUROPE DELAWARE, INC., a Delaware corporation (collectively, the "Guarantors"),
STYROCHEM EUROPE DELAWARE, INC, a Delaware corporation and RADNOR DELAWARE II,
INC., a Delaware corporation, (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 by a Fifth Supplemental
Indenture dated as of January 21, 1999, as amended by a Sixth Supplemental
Indenture dated as of March 23, 1999 (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 (the
"Guarantee") 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 Seventh 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 SEVENTH 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:

                                     2
<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 the New Guarantors were
the principal debtors, 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 each of the New Guarantors and shall be subject in all respects
to, and governed by all of the terms and provisions applicable to Guarantee 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) the New Guarantors.

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

                                               RADNOR HOLDINGS CORPORATION

Attest: /s/ Michael V. Valenza                    By: /s/ Michael T. Kennedy
       -----------------------                    --------------------------
        /s/ Michael V. Valenza                         Michael T. Kennedy
       Assistant Secretary                        President

                                               WINCUP HOLDINGS, INC.

Attest: /s/ Michael V. Valenza                    By: /s/ Michael T. Kennedy
       -----------------------                    --------------------------
       Michael V. Valenza                         Michael T. Kennedy
       Assistant Secretary                        President

                                               RADNOR CHEMICAL CORPORATION

Attest: /s/ Michael V. Valenza                    By: /s/ Michael T. Kennedy
       -----------------------                    --------------------------
       Michael V. Valenza                         Michael T. Kennedy
       Assistant Secretary                        Chairman

                                     3
<PAGE>

                                               RADNOR MANAGEMENT, INC.

Attest: /s/ Michael V. Valenza                    By: /s/ Michael T. Kennedy
       -----------------------                    --------------------------
       Michael V. Valenza                         Michael T. Kennedy
       Assistant Secretary                        President

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

Attest: /s/ Michael V. Valenza                    By: /s/ Michael T. Kennedy
       -----------------------                    --------------------------
       Michael V. Valenza                         Michael T. Kennedy
       Assistant Secretary                        Chairman

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

Attest: /s/ Michael V. Valenza                    By: /s/ Michael T. Kennedy
       -----------------------                    --------------------------
       Michael V. Valenza                         Michael T. Kennedy
       Assistant Secretary                        President

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

Attest: /s/ Michael V. Valenza                    By: /s/ Michael T. Kennedy
       -----------------------                    --------------------------
       Michael V. Valenza                         Michael T. Kennedy
       Assistant Secretary                        Chairman

                                     4
<PAGE>

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

Attest:/s/ Michael V. Valenza                     By: /s/ Michael T. Kennedy
       -----------------------                    --------------------------
       Michael V. Valenza                         Michael T. Kennedy
       Assistant Secretary                        Chairman

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

Attest: /s/ Michael V. Valenza                    By: /s/ Michael T. Kennedy
       -----------------------                    --------------------------
       Michael V. Valenza                         Michael T. Kennedy
       Assistant Secretary                        President

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

Attest: /s/ Michael V. Valenza                    By: /s/ Michael T. Kennedy
       -----------------------                    --------------------------
       Michael V. Valenza                         Michael T. Kennedy
       Assistant Secretary                        President

                                               RADNOR MANAGEMENT DELAWARE, INC.

Attest: /s/ Michael V. Valenza                    By: /s/ Michael T. Kennedy
       -----------------------                    --------------------------
       Michael V. Valenza                         Michael T. Kennedy
       Assistant Secretary                        President

                                               STYROCHEM DELAWARE, INC.

Attest: /s/ Michael V. Valenza                    By: /s/ Michael T. Kennedy
       -----------------------                    --------------------------
       Michael V. Valenza                         Michael T. Kennedy
       Assistant Secretary                        Chairman

                                     5
<PAGE>

                                               WINCUP EUROPE DELAWARE, INC.

Attest: /s/ Michael V. Valenza                 By: /s/ Michael T. Kennedy
       -----------------------                    --------------------------
       Michael V. Valenza                         Michael T. Kennedy
       Assistant Secretary                        Chairman

                                               STYROCHEM EUROPE DELAWARE, INC.

Attest: /s/ Michael V. Valenza                 By: /s/ Michael T. Kennedy
       -----------------------                    --------------------------
       Michael V. Valenza                         Michael T. Kennedy
       Assistant Secretary                        Chairman

                                               RADNOR DELAWARE II, INC.

Attest: /s/ Michael V. Valenza                 By: /s/ Michael T. Kennedy
       -----------------------                    --------------------------
       Michael V. Valenza                         Michael T. Kennedy
       Assistant Secretary                        Chairman

                                               FIRST UNION NATIONAL BANK,
                                               as Trustee

Attest: /s/ Ralph E. Jones                     By: /s/ Alan G. Finn
       -----------------------                    --------------------------
       Name:  Ralph E. Jones                      Name:   Alan G. Finn
       Title: AVP                                 Title:  Vice President

                                     6