<PAGE>

                                                                   Exhibit 10.23


                             ASSIGNMENT AGREEMENT

     This Assignment Agreement (the "Assignment") is entered into as of December
29, 1999 by and among GMAC COMMERICAL CREDIT LLC, formerly BNY Factoring LLC,
successor by merger to BNY Financial Corporation (the "U.S. Assignor"), GMAC
COMMERICAL CREDIT DEVELOPMENT LIMITED, formerly BNY Financial Limited (the "U.K.
Assignor" and, together with the U.S. Assignor, the "Assignors") and BANK OF
AMERICA, N.A., in its individual capacity and in its capacity as Lender under
the Loan Agreement described below, formerly NationsBank, N.A. ("Assignee").

                                   RECITALS
                                   --------

     WHEREAS, the U.S. Assignor is the Agent under the Second Amended and
Restated Revolving Credit and Security Agreement dated as of October 15, 1997
(as same has been or may be amended, supplemented or otherwise modified in
accordance with the terms thereof, the "Loan Agreement") among (i) Wincup
Holdings, Inc., Radnor Chemical Corporation (formerly known as SP Acquisition
Co.), StyroChem U.S. Ltd. (formerly known as StyroChem International, Inc.),
Radnor Holdings Corporation, Radnor Delaware, Inc., StyroChem Delaware, Inc. and
Wincup Texas, Ltd., as Borrowers (each, a "U.S. Borrower" and jointly and
severally, the "U.S. Borrowers"), (ii) the U.S. Assignor, the Assignee and each
of the other financial institutions named therein or which have become a party
to such agreement, as Lenders (the "U.S. Lenders") and (iii) the U.S. Assignor,
as administrative and collateral agent for the U.S. Lenders (the "U.S. Agent");

     WHEREAS, the U.K. Assignor is the Agent under the Supplement Revolving
Multicurrency Credit Agreement dated as of 12th December, 1997 (as same has been
or may be amended, supplemented or otherwise modified in accordance with the
terms thereof, the "European Supplement") among (i) StyroChem Europe (The
Netherlands) B.V., StyroChem Finland Oy, Thermisol Denmark A/S, Thermisol Sweden
AB and Thermisol Finland Oy, as Borrowers (each, a "European Borrower" and
jointly and severally, the "European Borrowers"; collectively, the U.S.
Borrowers and the European Borrowers are referred to herein as the "Borrowers"),
(ii) the U.K. Assignor, the Assignee and each of the other financial
institutions named therein or which have become a party to such agreement, as
Lenders (the "European Lenders") and (iii) the U.K. Assignor, as administrative
agent for the European Lenders (the "European Agent");

     WHEREAS, in connection with execution of the Loan Agreement and the
European Supplement, the Borrowers and other Credit Parties executed and
delivered certain documents, instruments and agreements to the Assignors as more
particularly set forth on Exhibit A hereof (collectively, the "Financing
                          ---------
Documents");

     WHEREAS, at the Borrowers' request, (i) the U.S. Assignor has agreed to
assign, and the Assignee has agreed to accept U.S. Assignor's rights and
obligations as U.S. Agent under the Loan Agreement, including without limitation
its rights in the Financing Documents, (ii) the U.K. Assignor has agreed to
assign, and the Assignee has agreed to accept, all of the U.K. Assignor's rights
and obligations as European Agent under the European Supplement, including
without limitation its rights in the Financing Documents, and (iii) the parties
hereto have agreed that the Assignee shall become the successor Agent under the
Loan Agreement, the European Supplement and the Other Documents referred to
therein; and

     WHEREAS, this Assignment is being executed and delivered in accordance with
Section 15.3 of the Loan Agreement and Section 15.3 of the European Supplement;

     NOW, THEREFORE, the parties hereto hereby agree as follows:

                                   AGREEMENT
                                   ---------

     1.   All capitalized terms used herein which are not defined shall have the
meanings given to them in the Loan Agreement and/or the European Supplement, as
applicable.

     2.   This Assignment shall become effective on the date hereof (the
"Transfer Effective Date"), and from and after the Transfer Effective Date, (a)
the Assignee shall be the Agent under each of the Loan Agreement
<PAGE>

and the European Supplement and for all purposes related thereto and (b) upon
simultaneously transferring their commitments, as Lenders, to First Union
National Bank, the Assignors shall no longer be parties to the Loan Agreement,
the European Supplement or the Other Documents, provided, however, Assignors
shall continue to retain their rights under the Loan Agreement and/or the
European Supplement as to any rights accruing to Assignors while they were
parties to such agreements, including any indemnity rights of Assignors which
survive the termination of the Loan Agreement pursuant to the terms thereof.

     3.  The Assignors have made arrangements with the Assignee with respect to
(a) the portion, if any, to be paid, and the date or dates for payment, by the
Assignors to the Assignee of any fees heretofore received by the Assignors, in
their capacity as agents, pursuant to the Loan Agreement and the European
Supplement prior to the Transfer Effective Date and (b) the portion, if any, to
be paid, and the date or dates for payment, by the Assignee to the Assignors of
fees or interest received by Assignee, in its capacity as agent, pursuant to the
Loan Agreement or the European Supplement from and after the Transfer Effective
Date.  Borrowers acknowledge and agree that to induce Assignors to execute this
Assignment and the Commitment Transfer Supplements, dated as of the date hereof,
with First Union National Bank, Borrowers shall pay to Assignors a fee equal to
$200,000.  The parties hereto acknowledge and agree that each Lender under the
Loan Agreement hereby waives its respective rights to any prepayment and/or
termination fee under the Loan Agreement.

     4.  All principal payments that would otherwise be payable from and after
the Transfer Effective Date to or for the account of an Assignor, in its
capacity as agent, pursuant to the Loan Agreement, the European Supplement
and/or the Notes shall, instead, be payable to or for the account of the
Assignee, as agent.

     5.  Concurrently with the execution and delivery hereof, the Assignors will
provide to the Assignee conformed copies of the Loan Agreement, the European
Supplement and all related documents delivered to the Assignors to the extent
such documents have not previously been delivered to the Assignee.

     6.  Each Assignor represents and warrants that it is a holder of the
Financing Documents being assigned by it hereunder and that its interest as
described in the Financing Documents to such Assignor's knowledge are free and
clear of any adverse claim.

     7.  Further, each Assignor represents and warrants to the Assignee as
follows:

         (a) such Assignor has not previously assigned or pledged any of its
     rights under any of the Financing Documents to any other person.

         (b) to the best of its knowledge, such Assignor is not in default under
     any of the Financing Documents.

         (c) such Assignor does not know of any default on the part of any other
     party to the Financing Documents (other than Bank of America, N.A.).

         (d) such Assignor is not prohibited under any agreement with any other
     person or under any judgment or decree from the execution and delivery of
     this Assignment.

         (e) such Assignor does not know of any action that has been brought or
     threatened which would in any way prohibit or impair the execution and
     delivery of this Assignment.

         (f) no consent or other approval is required in connection with the
     execution and delivery of this Assignment by such Assignor other than the
     consent of the Borrowers hereunder.

         (g) to the best of the Assignors' knowledge, Exhibit A sets forth each
                                                      ---------
     of the Financing Documents, together with any changes, extensions,
     revisions, modifications or guarantees of performance thereto, and true and
     correct copies of each of the Financing Documents have been delivered to
     the Assignee.
<PAGE>

          (h) such Assignor has full power to exercise any and all of its rights
     under any of the Financing Documents.

     8.   Each of the parties to this Assignment agrees that at any time and
from time to time upon the written request of any other party, it will execute
and deliver such further documents and do such further acts and things as such
other party may reasonably request in order to effect the purposes of this
Assignment at the Borrowers' cost.

     9.   By executing and delivering this Assignment, the Assignors and the
Assignee confirm to and agree with each other as follows: (a) other than the
representation and warranty that it is the legal and beneficial owner of the
interest being assigned hereby free and clear of any adverse claim and those set
forth in Section 7 hereof, no Assignor makes any representation or warranty or
assumes any responsibility with respect to any statements, warranties or
representations made in or in connection with the Loan Agreement or the European
Supplement or the execution, legality, validity, enforceability, genuineness,
sufficiency or value of the Loan Agreement, the European Supplement the Notes or
any other instrument or document furnished pursuant thereto; (b) no Assignor
makes any representation or warranty or assumes any responsibility with respect
to the financial condition of Borrowers or the performance or observance by
Borrowers of any of their Obligations under the Loan Agreement, the European
Supplement, the Notes or any other instrument or document furnished pursuant
thereto; (c) the Assignee confirms that it has received a copy of the Loan
Agreement and the European Supplement, together with copies of such financial
statements and such other documents and information as it has deemed appropriate
to make its own credit analysis and decision to enter into this Assignment; (d)
the Assignee will, independently and without reliance upon the Assignors and
based on such documents and information as it shall deem appropriate at the
time, continue to make its own credit decisions in taking or not taking action
under the Loan Agreement, the European Supplement or the Other Documents; (e)
the Assignee agrees that it will perform all of its respective obligations as
set forth in the Loan Agreement and the European Supplement to be performed by
it as a Lender and as Agent; and (f) the Assignee represents and warrants to the
Assignors and the Borrowers that it is either (i) entitled to the benefits of an
income tax treaty with the United States of America that provides for an
exemption from the United States withholding tax on interest and other payments
made by Borrowers under the Loan Agreement, the European Supplement and the
Other Documents or (ii) is engaged in trade or business within the United States
of America.

     10.  This Assignment shall be governed by, and construed in accordance
with, the laws of the State of New York.
<PAGE>

     IN WITNESS WHEREOF, the parties hereto have caused this Assignment to be
executed by their respective duly authorized officers on the date set forth
above.


                              GMAC COMMERCIAL CREDIT LLC, formerly BNY Factoring
                              LLC, successor by merger to BNY Financial
                              Corporation, as U.S. Assignor

                              By:/s/ Anthony Viola
                                 ------------------
                              Name:Anthony Viola
                              Title:Vice President


                              GMAC COMMERCIAL CREDIT DEVELOPMENT LIMITED,
                              formerly BNY Financial Limited, as U.K. Assignor

                              By:/s/ Peter Brady
                                 ----------------
                              Name:Peter Brady
                              Title:Senior Vice President


                              BANK OF AMERICA, N.A.,
                              as Assignee

                              By:/s/ David Noda
                                 ------------------
                              Name:David Noda
                              Title:Managing Director


Consented to:

FIRST UNION NATIONAL BANK

By:/s/ Constantin Chepurny
   ------------------------
Name:Constantin Chepurny
Title:Senior Vice President


WINCUP HOLDINGS, INC.

By:/s/ R. Radcliffe Hastings
   -------------------------
Name:R. Radcliffe Hastings
Title:Senior Vice President


RADNOR CHEMICAL CORPORATION

By:/s/ R. Radcliffe Hastings
   -------------------------
Name:R. Radcliffe Hastings
Title:Senior Vice President


STYROCHEM U.S., LTD.

By:/s/ R. Radcliffe Hastings
   -------------------------
Name:R. Radcliffe Hastings
Title:Senior Vice President
<PAGE>

RADNOR HOLDINGS CORPORATION

By:/s/ R. Radcliffe Hastings
   -------------------------
Name:R. Radcliffe Hastings
Title:Senior Vice President

RADNOR DELAWARE, INC.

By:/s/ R. Radcliffe Hastings
   -------------------------
Name:R. Radcliffe Hastings
Title:Senior Vice President

STYROCHEM DELAWARE, INC.

By:/s/ R. Radcliffe Hastings
   -------------------------
Name:R. Radcliffe Hastings
Title:Senior Vice President

WINCUP TEXAS, LTD.

By:/s/ R. Radcliffe Hastings
   -------------------------
Name:R. Radcliffe Hastings
Title:Senior Vice President
<PAGE>

STYROCHEM EUROPE (THE NETHERLANDS) B.V.

By:/s/ R. Radcliffe Hastings
   -------------------------
Name:R. Radcliffe Hastings
Title:Authorized Representative

STYROCHEM FINLAND OY

By:/s/ R. Radcliffe Hastings
   -------------------------
Name:R. Radcliffe Hastings
Title:Director

THERMISOL DENMARK A/S

By:/s/ R. Radcliffe Hastings
   -------------------------
Name:R. Radcliffe Hastings
Title:Director

THERMISOL SWEDEN AB

By:/s/ R. Radcliffe Hastings
   -------------------------
Name:R. Radcliffe Hastings
Title:Director

THERMISOL FINLAND OY

By:/s/ R. Radcliffe Hastings
   -------------------------
Name:R. Radcliffe Hastings
Title:Director

<PAGE>

                                   EXHIBIT A
                                   ---------

                              FINANCING DOCUMENTS

LANDLORD WAIVERS
----------------

1.   Landlord Waiver dated 4/10/98 between Radnor Center Associates and WinCup
     Holdings, Inc. for property at Suite 300, Building 3.
2.   Landlord Waiver dated 12/12/97 between Stone Mountain Industrial Park, Inc.
     and WinCup Holdings, Inc. for property at 1625 Litton Drive, Stone
     Mountain, GA
3.   Landlord Waiver dated 12/13/97 between Patricia M. Dunnell and WinCup
     Holdings, Inc. for property at 5355 Shawland Rd., Jacksonville, FL
4.   Landlord Waiver dated 6/22/98 between Hunt Brothers Leasing LLC and WinCup
     Holdings, Inc. for property at 205 Tamal Vista, Corte Madera, CA 94925
5.   Landlord Waiver dated 6/22/98 between Hunt Brothers Leasing LLC and WinCup
     Holdings, Inc. for property at 201 Tamal Vista, Corte Madera, CA, 94925
6.   Landlord Waiver dated 6/22/98 between Hunt Brothers Leasing LLC and WinCup
     Holdings, Inc. for property at 195 Tamal Vista, Corte Madera, CA, 94925
7.   Landlord's Subordination and Waiver (undated) between CK Airpark Associates
     and WinCup Holdings, Inc. for property at 2215 East University Drive,
     Phoenix, AZ 85034
8.   Landlord's Subordination and Waiver (undated) between CK Airpark Associates
     and WinCup Holdings, Inc. for property at 2093 East Magnolia Street,
     Phoenix, AZ 85034
9.   Landlord Waiver dated 12/18/95 between Etzioni Partners and WinCup
     Holdings, L.P. for property at 64 Brunswick Avenue, Edison, NJ
10.  Landlord Waiver dated 12/5/96 between Bradford Management Company of
     Dallas, Inc. and The Bank of New York Commercial Corporation for property
     at 4933 Pylon Street, Fort Worth, Texas

LOCKBOX ACCOUNT AGREEMENTS
--------------------------

1.   Direct Deposit Lockbox Service Agreement dated as of 1/18/86 between WinCup
     Holdings, L.P. and The Bank of New York
2.   Agreement Relating to Lockbox Services dated 1/18/96 between Wincup
     Holdings, L.P. and The Bank of New York Commercial Corporation
3.   Special Deposit Agreement dated 12/4/96 between Styrochem International,
     Inc. and The Bank of New York Commercial Corporation

UCC FINANCING STATEMENTS
------------------------
(See Schedule 1 attached hereto)

INTELLECTUAL PROPERTY
---------------------

1.   Trademark Collateral Security Agreement dated 12/5/96 between StyroChem
     International, Inc. and The Bank of New York Commercial Corporation
2.   Trademark Collateral Security Agreement dated 12/5/96 between WinCup
     Holdings, Inc. and The Bank of New York Commercial Corporation

OTHER
-----

1.   Collateral Assignment of Rights under the Stock Purchase Agreement and
     Environmental Escrow Agreement dated 12/5/96 between Radnor Holdings
     Corporation and The Bank of New York Commercial Corporation
2.   Guaranty dated 3/23/99 by Radnor Management Delaware, Inc. and Wincup
     Europe Delaware, Inc.
3.   Guaranty dated 1/21/99 by Styrochem GP, L.L.C., Styrochem LP, L.L.C.,
     Wincup GP, L.L.C. and Wincup
<PAGE>

     LP, L.L.C.
4.   Reaffirmation and Ratification Agreement dated 10/15/97 between WinCup
     Holdings, Inc., SP Acquisition Co., StyroChem International, Inc., Radnor
     Holdings Corporation, NationsBank, N.A. and BNY Financial Corporation
5.   StyroChem Finland Oy Security Agreement dated 12/12/97 between Styrochem
     Finland Oy and BNY Financial Limited
6.   Promissory Notes
     (a)  No. 1 dated 12/12/97, issued by Stryrochem Finland Oy to BNY Financial
          Limited for 6.5 million FIM
     (b)  No. 2 dated 12/12/97, issued by Stryrochem Finland Oy to BNY Financial
          Limited for 6.5 million FIM
     (c)  No. 3 dated 1/26/98, issued by Stryrochem Finland Oy to BNY Financial
          Limited for 5 million FIM
     (d)  No. 4 dated 2/27/98, issued by Stryrochem Finland Oy to BNY Financial
          Limited for 5 million FIM
     (e)  No. 5 dated 4/16/98, issued by Stryrochem Finland Oy to BNY Financial
          Limited for 6 million FIM
7.   Thermisol Finland Oy Security Agreement dated as of12/12/97 between
     Thermisol Finland Oy and BNY Financial Limited
8.   Promissory Notes
     (a)  No. 1 dated 12/12/97, issued by Thermisol Finland Oy to BNY Financial
          Limited for 4 million FIM
     (b)  No. 2 dated 12/12/97, issued by Thermisol Finland Oy to BNY Financial
          Limited for 4 million FIM
     (c)  No. 3 dated 6/22/99, issued by Thermisol Finland Oy to BNY Financial
          Limited for 7 million FIM
9.   Agreement executed 8/24/98 regarding priorities among business mortgages in
     connection with the merger of Epsilevy Oy into Thermisol Finland Oy
<PAGE>

                                  SCHEDULE 1
                                  ----------
                    UCC Financing Statements to be assigned

DEBTOR: WINCUP HOLDINGS, INC.
------

<TABLE>
<CAPTION>
---------------------------------------------------------------------------------------------------------------------
     JURISDICTION                    SECURED                            FILE DATE                 FILE NO.
     ------------                    -------                            ---------                 --------
                                      PARTY
                                      -----
---------------------------------------------------------------------------------------------------------------------
<S>                     <C>                                            <C>                       <C>
Arizona SOS             BNY Financial Corporation, as Agent                 2/21/95              858956
---------------------------------------------------------------------------------------------------------------------
California SOS          BNY Financial Corporation, as Agent                 9/5/97               9725161066
---------------------------------------------------------------------------------------------------------------------
Florida SOS             BNY Financial Corporation, as Agent                 9/5/97               970000200562
---------------------------------------------------------------------------------------------------------------------
DeKalb Co., GA          BNY Financial Corporation, as Agent                 10/6/97              441997-9936
---------------------------------------------------------------------------------------------------------------------
Illinois SOS            BNY Financial Corporation, as Agent                 9/5/97               3736294
---------------------------------------------------------------------------------------------------------------------
Caddo Parish, LA        BNY Financial Corporation, as Agent                 9/9/97               09-953158
---------------------------------------------------------------------------------------------------------------------
Mississippi SOS         BNY Financial Corporation, as Agent                 9/5/97               01138725
---------------------------------------------------------------------------------------------------------------------
Lauderdale Co., MS      BNY Financial Corporation, as Agent                 9/5/97               7784-97
---------------------------------------------------------------------------------------------------------------------
Missouri SOS            BNY Financial Corporation, as Agent                 9/5/97               2827176
---------------------------------------------------------------------------------------------------------------------
Lafayette Co., MO       BNY Financial Corporation, as Agent                 9/5/97               2888
---------------------------------------------------------------------------------------------------------------------
New Jersey SOS          BNY Financial Corporation, as Agent                 9/17/97              1791540
---------------------------------------------------------------------------------------------------------------------
Ohio SOS                BNY Financial Corporation, as Agent                 9/5/97               AN93974
---------------------------------------------------------------------------------------------------------------------
Madison Co., OH         BNY Financial Corporation, as Agent                 9/5/97               9700001705
---------------------------------------------------------------------------------------------------------------------
Pennsylvania SOC        BNY Financial Corporation, as Agent                 12/21/95             25000497
---------------------------------------------------------------------------------------------------------------------
Delaware Co., PA        BNY Financial Corporation, as Agent                 12/21/95             95-203140
 Prothonotary
---------------------------------------------------------------------------------------------------------------------
</TABLE>


DEBTOR:    RADNOR HOLDINGS CORPORATION
------

<TABLE>
<CAPTION>
---------------------------------------------------------------------------------------------------------------------
     JURISDICTION                     SECURED                            FILE DATE               FILE NO.
     ------------                     -------                            ---------               --------
                                       PARTY
                                       -----
---------------------------------------------------------------------------------------------------------------------
<S>                     <C>                                            <C>           <C>
California SOS          BNY Financial Corporation, as Agent                12/9/96             9634560484
---------------------------------------------------------------------------------------------------------------------
Illinois SOS            BNY Financial Corporation, as Agent                12/11/96            3624613
---------------------------------------------------------------------------------------------------------------------
Mississippi SOS         BNY Financial Corporation, as Agent                12/9/96             01058141
---------------------------------------------------------------------------------------------------------------------
Lauderdale Co., MS      BNY Financial Corporation, as Agent                12/10/96            8987-96
---------------------------------------------------------------------------------------------------------------------
Pennsylvania SOC        BNY Financial Corporation, as Agent                12/9/96             26140944
---------------------------------------------------------------------------------------------------------------------
Delaware Co., PA        BNY Financial Corporation, as Agent                12/10/96            96-203162
 Prothonotary
---------------------------------------------------------------------------------------------------------------------
</TABLE>


DEBTOR:    RADNOR DELAWARE, INC.

<TABLE>
<CAPTION>
---------------------------------------------------------------------------------------------------------------------
     JURISDICTION                          SECURED                      FILE DATE                FILE NO.
     ------------                          -------                      ---------                --------
                                            PARTY
                                            -----
---------------------------------------------------------------------------------------------------------------------
<S>                     <C>                                             <C>                      <C>
Delaware SOS            BNY Financial Corporation, as Agent                4/13/98                9815741
---------------------------------------------------------------------------------------------------------------------
</TABLE>
<PAGE>

DEBTOR:    STYROCHEM DELAWARE, INC.

<TABLE>
<CAPTION>
---------------------------------------------------------------------------------------------------------------------
     JURISDICTION                          SECURED                      FILE DATE                FILE NO.
     ------------                          -------                      ---------                --------
                                            PARTY
                                            -----
---------------------------------------------------------------------------------------------------------------------
<S>                     <C>                                             <C>                      <C>
Delaware SOS            BNY Financial Corporation, as Agent                 10/5/99               9951592
---------------------------------------------------------------------------------------------------------------------
</TABLE>