EX-4.2



 Exhibit 4.2 

JOINDER AGREEMENT 
 June 1, 2015 

Merrill Lynch, Pierce, Fenner & Smith 

                     Incorporated 

J.P. Morgan Securities LLC 
 As Representatives
of the several Initial Purchasers 
 c/o Merrill Lynch, Pierce, Fenner & Smith 

         Incorporated 

One Bryant Park 
 New York, New York 10036 

c/o J.P. Morgan Securities LLC 
 383 Madison Avenue 

New York, New York 10179 
 Reference is made to
that registration rights agreement (the “Registration Rights Agreement”) dated as of May 26, 2015 among A. Schulman, Inc., a Delaware corporation (the “Company”), the Schulman Guarantors, Merrill Lynch,
Pierce, Fenner & Smith and J.P. Morgan Securities LLC, as the Representatives, relating to the issuance and sale to the Initial Purchasers of $375,000,000 aggregate principal amount of the Company’s 6.875% Senior Notes due 2023
(the “Notes”). Capitalized terms used herein and not otherwise defined herein shall have the meanings ascribed to such terms in the Registration Rights Agreement. 

The Registration Rights Agreement contemplates that substantially concurrently with the consummation of the Acquisition, HGGC Citadel Plastics
Holdings, Inc. and each of its subsidiaries that guarantee the Notes will becomes parties to the Registration Rights Agreement by executing this Registration Rights Joinder. 

1. Joinder. Each of the undersigned hereby acknowledges that it has received and reviewed a copy of the Registration Rights Agreement
and all other documents it requires to enter into this Joinder Agreement (the “Registration Rights Joinder”), and acknowledges and agrees to (i) join and become a party to the Registration Rights Agreement as indicated by its
signature below; (ii) be bound by all covenants, agreements, representations, warranties and acknowledgements attributable to a Guarantor in the Registration Rights Agreement as if made by, and with respect to, each Guarantor signatory hereto;
and (iii) perform all obligations and duties required of a Guarantor pursuant to the Registration Rights Agreement. 
 2.
Counterparts. This Registration Rights Joinder may be signed in one or more counterparts (which may be delivered in original form or facsimile or “pdf” file thereof), each of which shall constitute an original when so executed and
all of which together shall constitute one and the same agreement. 

  
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 3. Amendments. No amendment or waiver of any provision of this Registration Rights
Joinder, nor any consent or approval to any departure therefrom, shall in any event be effective unless the same shall be in writing and signed by the parties thereto. 

4. Headings. The section headings used herein are for convenience only and shall not affect the construction hereof. 

5. APPLICABLE LAW. THIS REGISTRATION RIGHTS JOINDER SHALL BE GOVERNED BY AND CONSTRUED IN ACCORDANCE WITH THE LAWS OF THE STATE OF NEW
YORK, WITHOUT REGARD TO THE CONFLICTS OF LAW RULES THEREOF. 

  
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 IN WITNESS WHEREOF, each of the undersigned has caused this Registration Rights Joinder to be
duly executed and delivered, by its proper and duly authorized officer as of the date set forth above. 
  

					
	HGGC CITADEL PLASTICS HOLDINGS, INC.
		
	By: 		 /s/ David C. Minc

			Name: 		David C. Minc
			Title:		Secretary
	
	HGGC CITADEL PLASTICS INTERMEDIATE HOLDINGS, INC.
		
	By:		 /s/ David C. Minc

			Name:		David C. Minc
			Title:		Secretary
	
	CITADEL PLASTICS HOLDINGS, INC.
		
	By:		 /s/ David C. Minc

			Name:		David C. Minc
			Title:		Secretary
	
	CITADEL INTERMEDIATE HOLDINGS, LLC
		
	By:		 /s/ David C. Minc

			Name:		David C. Minc
			Title:		Secretary
	
	THE MATRIXX GROUP, INCORPORATED
		
	By:		 /s/ David C. Minc

			Name:		David C. Minc
			Title:		Secretary



  
 [Signature Page to
Joinder to Registration Rights Agreement] 








 
					
	BULK MOLDING COMPOUNDS, INC.
		
	By: 		 /s/ David C. Minc

			Name: 		David C. Minc
			Title:		Secretary
	
	HPC HOLDINGS, LLC
		
	By:		 /s/ David C. Minc

			Name:		David C. Minc
			Title:		Secretary



  
 [Signature Page to
Joinder to Registration Rights Agreement]