DETERMINATION OF THE EFFECT OF EXTRACTED OXYGEN HETEROCYCLE ON THE DEHYDROCHLORINATION OF VI NYLIDENE CHLORIDE COPOLYMER

1.1Background of study

Vinylidine chloride, also referred to as 1,1-dichloroethylene (1,1-DCE) is a colourless liquid (b.p.  32.2qC),  produced  by  dehydrochlorination  of  1,1,2  ±trichloroethane  [Cl2CH CH2Cl],  a relatively  unwanted  by-product  formed  in  the  production  of  1,1,1±trichloroethane  and  1,2± dichloroethane. Conversion to 1,1±DCE involves a base ±catalyzed reaction

Cl Cl Cl

 

 

 

H C C

 

H + C CH2

 

+ NaCl +

 

H2O

 

 

Cl H Cl

1, 1, 2-trichloroethane 1, 1-dichloroethylene (Rossberg et al., 2006)

 

1,1±DCE is mainly used as a comonomer in the polymerization of vinyl chloride, acrylonitrile, and acrylates. Inhibitors such as the monomethyl ether of hydroquinone are usually added to prevent  the  polymerization  of  1,1±DCE  on  storage.  1,1  ±DCE  is  also  used  in  semi  conductor device  fabrication  for  growing  high  purity  silicon  dioxide  (SiO2)  films.  1,1±DCE  is  thus  an important chemical substance which serves as a solvent as well as a monomer (Rossberg et al., 2006).

1,1±DCE  is  considered  a  potential  occupational  carcinogen  by  the  US  National  Institute  for Occupational Safety and Health.

As  with  several  other  alkenes,  1,1±DCE  can  be  polymerized  to  form  the  homopolymer poly(vinylidene chloride), PVDC, a form that is not commercially important because experimental data have been generated which demonstrated unequivocally that it undergoes what has  been  described  as  catastrophic  decomposition  at  its  melt  temperature  (above  125qC),

producing HCl (Piringer and Baner, 2008; Marianne, G., 2017). 1,1 ±DCE as monomer forms copolymers with other monomers such as vinyl chloride and these copolymers are commercially viable (Paisley, 2007).

Polyvinylidene chloride resins and coatings have reportedly been a part of flexible packaging for sometime. They are presented as possessing a unique combination of functional characteristics which have made them find numerous applications. According to Paisley, (2007) PVDC products are available in a variety of forms, such as aqueous dispersions or latex, for coating on a number of different film and paper substrates, extrudable powders for production of multilayer films and sheets, and soluble powders for solvent ±based coatings. Unique properties possessed by PVDC copolymers enable their use for protection from moisture loss or gain, protection against oxidation of ingredients, prevention of oil and grease permeation, and excellent printing characteristics(Michel, 2013). Paisley, (2007) is also on record as asserting that PVDC ±coated biaxially-oriented polypropylene (BOPP) films held, as at 2005, a 53% share of barrier film use in the USA. Such a figure does indicate the extensive use of PVDC products and therefore of their importance.

Again according to Paisley, (2007), during production of PVDC, other comonomers/additives are usually introduced, depending on the quality of copolymer desired. Thus, one comonomer/additive may be introduced to improve heat processability by decreasing melt temperature or to enable suitability of the polymer for film production, while another is introduced to provide some desired properties that would enhance polymer aesthetics, such as printability, adhesion, and / or thermoforming shrink flexibility. Thus several of these comonomers/additives may be added depending on desired effects. Additives are common in general polymer use. For instance, plasticizers are used to make PVC more flexible and therefore

 

more easily processed. On the other hand, terpolymers are increasingly in use, and include the acrylonitrile - butadiene - styrene (ABS) system which is generally formulated as an unbranched, head - to - tail terpolymer in which the individual comonomers are statistically distributed (Lewis, 1993).

Details of the microstructure of PVDC copolymers on the other hand are not yet available but it is reasonable to speculate that the two bulky chlorine atoms on one carbon atom joined to neighbouring carbon by a double bond in the monomer 1,1- DCE would provide sufficient steric hindrance to free rotation such that the relative stereochemistry in the resulting PVDC copolymer would not be random but would be predominantly syndiotactic leading to crystallinity to a greater degree. Thus PVDC copolymers are generally high density and high crystallinity with relatively few defect sites. High vinylidene chloride (VDC) content copolymers do however undergo thermally-induced degradative dehydrochlorination at process temperatures, and these degradations have been of interest ever since such copolymers came into use (Matheson and Boyer, 1952).

The dehydrochlorination process has more recently come under close scrutiny, and has been studied using largely thermogravimetric techniques (Howell et al., 2000).

Data generated in these thermogravimetric studies have been used to propose that only HCl is lost during degradation, and that weight loss of a PVDC copolymer sample directly indicates the extent of degradation which is usually represented as follows:

 

 

H

fast

 

H Cl

 

* C *

¢

H

 

* C C *

 

+ nHCl

 

This is the primary degradation process which accompanies the processing of the polymer. The early stage of dehydrohalogenation is uncomplicated by interfering processes, and the only product observed by evolved gas analysis is hydrogen chloride (Howell and Rajaram, 1993; Yue and Economy, 2017).