**PVB Sheet Recycling and Degradation**

Michael Tupý1, Dagmar Měřínská1,2 and Věra Kašpárková2,3

*<sup>1</sup>Department of Polymer Engineering, Faculty of Technology, Tomas Bata University in Zlin, Zlin, <sup>2</sup>Centre of Polymer Systems, University Institute, Tomas Bata University in Zlin, Zlin, <sup>3</sup>Department of Fat, Surfactant and Cosmetics Technology, Faculty of Technology, Tomas Bata University in Zlin, Zlin, Czech Republic*

#### **1. Introduction**

132 Material Recycling – Trends and Perspectives

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Increasing growth of raw material prices, environmental aspects and still growing landfill fees bring about the increasing interest encountered with the plastics waste recycling. Globally, the problem has been solved for the common plastics such as polyolefins, poly(ethylene terephtalate) and poly(styrene). Though extensively used for the glass lamination, poly(vinyl butyral) (PVB) does not belong to this group. It is generally known that during the glass lamination process, large volume of PVB trim is formed.

The PVB polymer is mostly used in the form of plasticized PVB sheet for preparation of laminated safety glass (LSG). At present time, Solutia, DuPont, Seki sui and Kuraray are worldwide PVB manufacturers. The PVB is the material which can stick together float glasses with holding excellent optical and mechanical properties of the glass laminate (Ivanov, 2006; v. Elasticity, mechanical strength, toughness, high light transmission and the adhesion to glass are the most significant PVB properties (Tupý, Měřínská, et al, 2010). Toughness of PVB sheet is based on high molecular weight of PVB chain. Certainly, the PVB has to be plasticized for achieving high material elasticity (Iwasaki, et al, 2006; Keller, Mortelmans, 1999; Svoboda, Balazs., et al, 1988). Admittedly, the plasticizer must not reduce light transmittance through the sheet, PVB adhesion to glass, generate haze and yellowness, and migrate out of the polymer matrix (Wade, D'Errico, et al, 2004). In addition, plasticizer must have perfect compatibility with the polymer and low evaporability during processing conditions.

#### **2. Sources for recycling of PVB sheet**

Worldwide, 65% of all PVB sheets are used in automotive applications (Dhaliwal, Hay, 2002). According to data from (OCIA, 2007), the worldwide car's production is estimated around 60 millions cars per a year. Assuming that one windshield contains approx. 1 kg of PVB sheet, total amount gives between 60-70 million kg of PVB sheets per year. In addition, by-products from PVB sheets manufacturing (5%) and trimmings (< 10%) from windshield production must be added (Goroghovski, Escapante-Garcia, et al, 2005). It represents total amount of 80 million kg of automotive PVB sheet waste annually. Total worldwide amount of produced PVB sheets for automotive and architectural industry is estimated around 120 million kg per year.

PVB Sheet Recycling and Degradation 135

physical properties as extruded material before the lamination process and it can be re-

The way assigned for PVB production is not easy. Firstly, it is necessary to produce poly(vinyl acetate) (PVAc) by the radical vinyl acetate polymerization. Consecutive hydrolysis at acidic or basic ambient creates poly(vinyl alcohol) (PVAl) which provides poly(vinyl butyral) by acetalization with butyraldehyde at acidic environment. The final structure of high-molecular PVB is used for LSG manufacturing and it is compounded from atactic copolymers 80% of vinyl butyral, 18-23% of vinyl alcohol and up to 1% of vinyl acetate [Wade, D'Errico, et al, 2004; Dhaliwal, Hay, 2002; D'Errico, Jemmott, et al, 1996; Nghuen, Berg, 2004; Svoboda, et al., 1970). This chemical structure, viewed in Fig.1, is the same for every manufacturer today. Nevertheless, exact consequence and properties of every PVB sheet depends on every PVB type, manufacturer and PVB sheet composition. The sheet is mostly stabilized by antioxidants

Fig. 1. PVB chain structure with composition of functional groups: vinyl butyral 81%, vinyl

Final PVB properties are assigned by polymerization degree of input PVAc, distribution curve of molecular weight, PVAl hydrolysis degree, catalyst acid strength, reaction temperature and PVAl conversion degree to PVB. The last one is crucial for final polarity of produced PVB. The PVB polymer is white powder, dissolvable in ethanol, THF, ketones and other semi-polar dissolvent (Mrkvičková, Daňhelka, 1984). The PVB solubility depends on – OH group content in the polymer chain and PVB molecular weight (Physical prop.SekiSui,

Certainly, achievement of high PVB toughness must be proved by polymer plasticizing. However, the plasticizer must not considerably reduce light transmittance through the sheet, increase haze and yellowness, reduce PVB adhesion to glass and migrate out of the polymer matrix (Svoboda, Balazs, et al, 1988; Zvoníček, 1999). The plasticizer also must have perfect compatibility with polymer matrix and its evaporability during the processing is prohibited (Zvoníček, 1999). Different plasticizers, used in beyond, were for example triethylenglycol-di-2-ethylbutyrate, dibuthylsebacate, tetraethylenglycol-di-heptanoate and

extruded and re-laminated to new PVB sheet again.

and thermal stabilizers in many times (Saflex, 1993).

alcohol 18-23%; vinyl acetate <1%.

2001).

**3.2 PVB sheet** 

**3. PVB sheet composition** 

**3.1 Poly(vinyl butyral)** 

#### **2.1 By-product from sheet manufacturing process**

This kind of PVB sheet has the best quality for following recycling process. By-product sheets are not contaminated by powder fragments but there may be present some unhomogeneous parts like plasticizer, light and heat stabilizers, adhesion modifiers, pigments and other elements of the system. The reasons why the sheet has to be re-processed are some specified defects in manufacturing technology (thickness, sheet roughness, edges). In every case, sheet edges must be cut approx. 10-20 cm from the edge beginning. The PVB sheet with a width around 2-3 meters has very high material shrinkage in this place. The edges can be replaced back to the extrusion process for re-extruding (Zvoníček, 1999).

#### **2.2 Trim**

The first step in LSG manufacturing is the lamination of PVB sheet between two glasses. It this part, probable defects on laminate edges must be reduced. It is performed by layering of PVB sheet between glasses with a larger sheet surface than the glass sizes are. Prepared "sandwich" is fixed by nipp-roll pre-lamination process and consequently it is possible to trim off oversized PVB sheet. While the sheet oversize would not be used, the laminate would be produced with visible defects (it does not meet the quality specifications) (Svoboda, Balazs., et al, 1988; Zvoníček, 1999).

The quantity of a trim generation depends on the geometry of produced windshield and the geometry of used PVB sheet surface. The width of trim obtained from the pre-laminating process is around 1–20 cm, based on glass geometry. Therefore, the trim quantity is between 5-10% from total amount of processed PVB sheet and the worldwide PVB trim capacity obtained from the windshield laminating process is approx. 4-6 million kg (Svoboda, Balazs., et al, 1988; Zvoníček, 1999).

The trim is created also at manufacturing of architectural LSG. Nevertheless, the ratio of a collected architectural trim is lower than amount of automotive trim. It is influenced by a higher glass powder contamination of this trim (different trimming technology) (Zvoníček, 1999).

### **2.3 PVB sheet from windshield**

Some specialized companies deal with the re-application possibility of PVB sheet obtained from recycled windshield. All present recycling processes produce a good quality of glass scrap which is fully re-used in the glass batch. However, the separated PVB waste is not recyclable due to high amount of glass, water content in the sheet, parts of color PVB sheets, mixed of various PVBs (after blending a haze is created) and foreign plastic matters (Zvoníček, 1999; Plaček, 2006; Recycling .., 2007; Tupý, Měřínská, 2011). Nevertheless, if somebody would develop the recycling technology ensures high-grade of glass separation, obtained PVB sheet may be re-processable to new PVB sheet without optical defects (Tupý, Měřínská, 2011).

Moreover, it is necessary to remain that the laminated (interglassed) PVB is not essentially exposed to UV radiation (up to 320 nm), mechanical stress, elevated temperature, oxygen and any various substances. Thus, the interglassed PVB sheets should keep very similar physical properties as extruded material before the lamination process and it can be reextruded and re-laminated to new PVB sheet again.

#### **3. PVB sheet composition**

#### **3.1 Poly(vinyl butyral)**

134 Material Recycling – Trends and Perspectives

This kind of PVB sheet has the best quality for following recycling process. By-product sheets are not contaminated by powder fragments but there may be present some unhomogeneous parts like plasticizer, light and heat stabilizers, adhesion modifiers, pigments and other elements of the system. The reasons why the sheet has to be re-processed are some specified defects in manufacturing technology (thickness, sheet roughness, edges). In every case, sheet edges must be cut approx. 10-20 cm from the edge beginning. The PVB sheet with a width around 2-3 meters has very high material shrinkage in this place. The edges can be replaced back to the extrusion process for re-extruding (Zvoníček, 1999).

The first step in LSG manufacturing is the lamination of PVB sheet between two glasses. It this part, probable defects on laminate edges must be reduced. It is performed by layering of PVB sheet between glasses with a larger sheet surface than the glass sizes are. Prepared "sandwich" is fixed by nipp-roll pre-lamination process and consequently it is possible to trim off oversized PVB sheet. While the sheet oversize would not be used, the laminate would be produced with visible defects (it does not meet the quality specifications)

The quantity of a trim generation depends on the geometry of produced windshield and the geometry of used PVB sheet surface. The width of trim obtained from the pre-laminating process is around 1–20 cm, based on glass geometry. Therefore, the trim quantity is between 5-10% from total amount of processed PVB sheet and the worldwide PVB trim capacity obtained from the windshield laminating process is approx. 4-6 million kg (Svoboda,

The trim is created also at manufacturing of architectural LSG. Nevertheless, the ratio of a collected architectural trim is lower than amount of automotive trim. It is influenced by a higher glass powder contamination of this trim (different trimming technology) (Zvoníček,

Some specialized companies deal with the re-application possibility of PVB sheet obtained from recycled windshield. All present recycling processes produce a good quality of glass scrap which is fully re-used in the glass batch. However, the separated PVB waste is not recyclable due to high amount of glass, water content in the sheet, parts of color PVB sheets, mixed of various PVBs (after blending a haze is created) and foreign plastic matters (Zvoníček, 1999; Plaček, 2006; Recycling .., 2007; Tupý, Měřínská, 2011). Nevertheless, if somebody would develop the recycling technology ensures high-grade of glass separation, obtained PVB sheet may be re-processable to new PVB sheet without optical defects (Tupý,

Moreover, it is necessary to remain that the laminated (interglassed) PVB is not essentially exposed to UV radiation (up to 320 nm), mechanical stress, elevated temperature, oxygen and any various substances. Thus, the interglassed PVB sheets should keep very similar

**2.1 By-product from sheet manufacturing process** 

(Svoboda, Balazs., et al, 1988; Zvoníček, 1999).

Balazs., et al, 1988; Zvoníček, 1999).

**2.3 PVB sheet from windshield** 

**2.2 Trim** 

1999).

Měřínská, 2011).

The way assigned for PVB production is not easy. Firstly, it is necessary to produce poly(vinyl acetate) (PVAc) by the radical vinyl acetate polymerization. Consecutive hydrolysis at acidic or basic ambient creates poly(vinyl alcohol) (PVAl) which provides poly(vinyl butyral) by acetalization with butyraldehyde at acidic environment. The final structure of high-molecular PVB is used for LSG manufacturing and it is compounded from atactic copolymers 80% of vinyl butyral, 18-23% of vinyl alcohol and up to 1% of vinyl acetate [Wade, D'Errico, et al, 2004; Dhaliwal, Hay, 2002; D'Errico, Jemmott, et al, 1996; Nghuen, Berg, 2004; Svoboda, et al., 1970). This chemical structure, viewed in Fig.1, is the same for every manufacturer today. Nevertheless, exact consequence and properties of every PVB sheet depends on every PVB type, manufacturer and PVB sheet composition. The sheet is mostly stabilized by antioxidants and thermal stabilizers in many times (Saflex, 1993).

Fig. 1. PVB chain structure with composition of functional groups: vinyl butyral 81%, vinyl alcohol 18-23%; vinyl acetate <1%.

Final PVB properties are assigned by polymerization degree of input PVAc, distribution curve of molecular weight, PVAl hydrolysis degree, catalyst acid strength, reaction temperature and PVAl conversion degree to PVB. The last one is crucial for final polarity of produced PVB. The PVB polymer is white powder, dissolvable in ethanol, THF, ketones and other semi-polar dissolvent (Mrkvičková, Daňhelka, 1984). The PVB solubility depends on – OH group content in the polymer chain and PVB molecular weight (Physical prop.SekiSui, 2001).

#### **3.2 PVB sheet**

Certainly, achievement of high PVB toughness must be proved by polymer plasticizing. However, the plasticizer must not considerably reduce light transmittance through the sheet, increase haze and yellowness, reduce PVB adhesion to glass and migrate out of the polymer matrix (Svoboda, Balazs, et al, 1988; Zvoníček, 1999). The plasticizer also must have perfect compatibility with polymer matrix and its evaporability during the processing is prohibited (Zvoníček, 1999). Different plasticizers, used in beyond, were for example triethylenglycol-di-2-ethylbutyrate, dibuthylsebacate, tetraethylenglycol-di-heptanoate and

PVB Sheet Recycling and Degradation 137

sheet. The PVB melt retains its shape, thickness and specific sheet surface which are necessary for de-aeration at pre-lamination process (Svoboda, Balazs, 1998; Zvoníček, 1999). Plasticizer and additives addition runs at mixing equipments before the extruder in many

The PVB sheet is mostly manufactured at the thickness 0.38 mm (LSG for architectonical industry) and 0.76 mm (LSG for automotive and architectonical use). Special applications require thicknesses 1.14 and 1.52 mm. The width of PVB sheet can be up to 3.5 meters. Manufactured PVB sheet is rolled and either separated by thin patterned polyethylene sheet or rolled under-cooled (Tg = 15°C) because the PVB sheet must not be stuck for following

Shear and thermo oxidative degradation of polymer represents more serious problem observed during reprocessing. Both degradation types induce the cleavage of polymer chains, albeit the degradation mechanism is not the same. Generally, the shortening of polymer chains negatively influences mechanical properties of PVB, resulting in an undesirable lowering of safety characteristics of the produced sheet. The deterioration of mechanical properties of PVB consequently decreases its ability to absorb the mechanical energy (when it is used for the safety car glass) in the case of an accident (Tupý, Zvoníček, 2008; Tupý, Měřínská, 2010). On the other hand, a decreasing of molecular weight and a decreasing of viscosity caused by the degradation can favorably influence rheological properties of PVB melt during the extrusion on the flat die (Měřínská, Tupý, 2010; Grachev,

In order to decrease power consumption during re-processing, PVB hygroscopicity can be utilized. Because water contained in PVB matrix can act as an additional plasticizer and lower the rigidity of the material, processing of "wet" PVB can be advantageous (Mrkvičková, Daňhelka, 1984). However, during the re-processing of material containing high moisture content (8 %), water can react with butyric groups, which induces the change of the polymer structure. As the consequence, hydrolysis occurs significantly changing the final properties of the re-processed PVB (Dhaliwal, Hay, 2002; Měřínská, Tupý, 2010).

The aim of the following test was to determine the degradation of PVB sheet at different kneading conditions and to estimate an influence of temperature, air oxygen content and mechanical stress on the course of degradation process. The work is also focused on the possibility to find optimal re-processing conditions of PVB whereat the mechanical and

First of all, the relative thermal stability of commercial PVB sample was measured by thermogravimetric analysis, from mass loss against temperature plots. As the Fig.2 shows, weight loss occurred in two distinct regions between 175–325 and 325–500 °C and corresponding to about 27-28 and 65-70% mass loss. No marked differences were observed between the various commercial grades. The first part of temperature range is the plasticizer evaporation. The final weight loss was the same for all the samples and a brown residue, approximately 5% of the original mass (Tupý, Měřínská, 2011). The evolved volatiles were analyzed by mass spectrometer as a function of time and temperature at fixed m/z ratios.

thermal degradation as well as yellowness of the re-processed polymer are minimal.

**5. PVB sheet degradation by thermogravimetric analysis** 

use (Svoboda, Balazs, 1998; Zvoníček, 1999; Saflex, 1973).

times.

Klimenko, et al, 1974).

dihexyladipate (Dhaliwal, Hay, 2001; Zvoníček, 1999). Mentioned plasticizers have different molecular polarity. There through, due to this it was indispensable to produce PVBs with different amount of hydroxyl groups in PVB chain (Dhaliwal, Hay, 2001; D'Errico, Jemmott, et al, 1996; Phillips, 2005). The final polymer system has a different absorbability of the plasticizer and water (Mrkvičková, Daňhelka, 1984). At present time, produced PVB sheets assigned for the glass lamination are plasticized with 28% of applied plasticizer; mostly triethylenglycole-bis(2-ethylhexanoete), (labeled 3GO) (Wade, D'Erricco, 2004; Phillips, 2005; Mister, Bianchi, et al, 2007; Smith, Rymer, et al, 2008).

Due to mentioned sheet physical properties, the most important characteristics of windshield are high mechanical strength and absorbability of kinetic energy during carcrash (Keller, Mortelmans, 1999; Svoboda, Balazs, 1988). However, PVB interlayer must keep glass particles on its surface [2, 3]. All these described properties provide an exactly adjusted adhesion grade of PVB to glass. Because the PVB has exceedingly high adhesion degree it cannot be used for automotive glass lamination. Thus, high adhesion degree it is necessary to reduce to ½ of the original adhesion value (Keller, Mortelmans, 1999; Wade, D'Erricco, 2004; Dhaliwal, Hay, 2004; Smith, Rymer, et al, 2008). The virgin adhesion is reduced by an addition of organic salts of alkali metals or alkaline-earth metals during an extrusion process of plasticized PVB sheet. This is described in several patents (Smith, Rymer, et al, 2008; Aoshima, Shohi, 2000; D'Errico, 1995; D'Erroco., 1997; Fowkes, 1987 Herman, Fabian, et al, 1984; Shichiri, Miyai, et al, 2002). The mostly used substances modifying the adhesion are organic salts of Na+, K+ and Mg2+. Ion ratio and its total amount are strictly specified (Smith, Rymer, et al, 2008; Aoshima, Shohi, 2000; D'Errico, 1995; D'Errico., 1997; Fowkes, 1987 Herman, Fabian, et al, 1984; Shichiri, Miyai, et al, 2002). Moreover, the water content in PVB sheet must be in range 0.3-0.5% which is necessary for a maintaining the required adhesion grade (Keller, Mortelmans, 1999; Wade, D'Errico, 2004; D'Errico, Jemmot, 1995; Shichiri, Miyai, 2002).
