<PAGE>

                                                                   Exhibit 10.35
                                    JOINDER


     Radnor Delaware II, Inc., a Delaware corporation (the "New Credit Party"),
hereby joins in the Fourth Amended and Restated Revolving Credit and Security
Agreement dated as of December 26, 2001 ("Credit Agreement") between WinCup
Holdings, Inc., Radnor Chemical Corporation, Radnor Holdings Corporation,
StyroChem U.S., Ltd., StyroChem Delaware, Inc., WinCup Texas, Ltd., StyroChem
GP, L.L.C., StyroChem LP, L.L.C., WinCup GP, L.L.C. and WinCup LP, L.L.C. (each,
a "Borrower" and jointly and severally, the "Borrowers"), the financial
institutions which are or became parties thereto (collectively, the "Lenders")
and PNC Bank, National Association, as agent of the Lenders (in such capacity,
the "Agent") as a "Borrower" thereunder and agrees to assume in full and become
jointly and severally liable for all of the Obligations (as defined in the
Credit Agreement) of the Borrowers thereunder and under any other agreement
between Borrowers, Agent and Lenders. Without limiting the generality of the
foregoing, in order to secure the prompt payment and performance to Agent and
Lenders of the Obligations, the New Credit Party hereby assigns, pledges and
grants to Agent a continuing security interest in and to all of its Collateral
(as defined in the Credit Agreement), whether now owned or existing or hereafter
acquired or arising and wheresoever located.  The undersigned further agrees to
deliver to Agent each of the documents described on Exhibit A attached hereto.

     IN WITNESS WHEREOF, and intending to be legally bound hereby, the New
Credit Party has duly executed this Joinder as of the 24/th/ day of January,
                                                      ------        --------
2002.



                                   RADNOR DELAWARE II, INC.



                                   By: /s/ Michael T. Kennedy
                                       ------------------------
                                   Name: Michael T. Kennedy
                                   Title:  Chairman

                                       1
<PAGE>

                          ACKNOWLEDGEMENT OF JOINDER

     The undersigned parties to the Credit Agreement hereby acknowledge and
agree to the joinder of Radnor Delaware II, Inc., as a Borrower under the Credit
Agreement.

IN WITNESS WHEREOF, and intending to be legally bound hereby, each of the
undersigned has duly executed this acknowledgement as of the 24/th/ day of
                                                             ------
January, 2002.
-------


                              WINCUP HOLDINGS, INC.


                              By: /s/ Michael T. Kennedy
                                  -------------------------------
                                  Michael T. Kennedy, President


                              RADNOR CHEMICAL CORPORATION


                              By: /s/ Michael T. Kennedy
                                  -------------------------------
                                  Michael T. Kennedy, Chairman


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


                              By: /s/ Michael T. Kennedy
                                  -------------------------------
                                  Michael T. Kennedy, Chairman


                              RADNOR HOLDINGS CORPORATION


                              By: /s/ Michael T. Kennedy
                                  -------------------------------
                                  Michael T. Kennedy, President


                              STYROCHEM DELAWARE, INC.


                              By: /s/ Michael T. Kennedy
                                  -------------------------------
                                  Michael T. Kennedy, Chairman

                                       2
<PAGE>

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


                              By: /s/ Michael T. Kennedy
                                  -------------------------------
                                    Michael T. Kennedy, President


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


                              By: /s/ Michael T. Kennedy
                                  -------------------------------
                                    Michael T. Kennedy, Chairman


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


                              By: /s/ Michael T. Kennedy
                                  -------------------------------
                                    Michael T. Kennedy, Chairman


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


                              By: /s/ Michael T. Kennedy
                                  -------------------------------
                                    Michael T. Kennedy, President


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


                              By: /s/ Michael T. Kennedy
                                  -------------------------------
                                    Michael T. Kennedy, President

                                       3
<PAGE>

                                    PNC BANK NATIONAL ASSOCIATION, as Lender



                                    By: /s/Janeann Fehrle, VP
                                        ---------------------------
                                    Name: Janeann Fehrle
                                    Title: VP

                                       4
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                                   Exhibit A


     (1) Schedules to the Credit Agreement updated to reflect the joinder of the
New Credit Party, as an additional Borrower thereto.

     (2) A copy of the resolution in form and substance reasonably satisfactory
to Agent, of the Board of Directors of the New Credit Party authorizing the
execution, delivery and performance of the joinder, the Credit Agreement and the
granting Liens upon its Collateral, certified by the Secretary or an Assistant
Secretary of the New Credit Party, as of the date of this joinder.

     (3) A copy of the Certificate of Incorporation of the New Credit Party, and
all amendments thereto, certified by the Secretary of State or other appropriate
official of its jurisdiction of incorporation with copies of the By-Laws of the
New Credit Party, certified as accurate and complete by the Secretary or an
Assistant Secretary of the New Credit Party.

     (4) A good standing certificate for the New Credit Party, dated not more
than thirty (30) days prior to the date of this joinder, issued by the Secretary
of State or other appropriate official of the New Credit Party's jurisdiction of
incorporation and each jurisdiction where the conduct of the New Credit Party's
business activities or the ownership of its properties necessitates
qualification.

     (5) Executed legal opinion of Duane, Morris & Heckscher LLP, counsel to the
New Credit Party, in form and substance satisfactory to Agent regarding the due
authorization, enforceability and validity of the joinder and the transactions
contemplated herein.

     (6) Any documents, agreements or instruments necessary, as determined by
Agent, to continue the effectiveness and maintain the priority of the security
interests in all Collateral that was assigned or transferred from Radnor
Delaware, Inc. to another entity.

                                       5