Cross-linking PVAC

Structural panels with outer facings, skins, fire retardants, and cores with foam are glued with EPI adhesives [1,4]. Well formulated EPI adhesives have shown very good wetting and adhesion properties to metals and due to the good processing properties EPI adhesives have taken over some of the epoxy, urethane and cross-linked PVAc markets [1], especially in the USA. 

Other Polymers. Besides polycarbonates, poly(methyl methacrylate)s, cycfic polyolefins, and uv-curable cross-linked polymers, a host of other polymers have been examined for their suitabiUty as substrate materials for optical data storage, preferably compact disks, in the last years. These polymers have not gained commercial importance polystyrene (PS), poly(vinyl chloride) (PVC), cellulose acetobutyrate (CAB), bis(diallylpolycarbonate) (BDPC), poly(ethylene terephthalate) (PET), styrene—acrylonitrile copolymers (SAN), poly(vinyl acetate) (PVAC), and for substrates with high resistance to heat softening, polysulfones (PSU) and polyimides (PI). 

However, predicting the molecular weight of the hydrolysis product is another matter. Most commercial PVAc is somewhat cross-linked. The cross-... 

SMP based on miscible blends of semicrystalline polymer/amorphous polymer was reported by the Mather research group, which included semicrystalline polymer/amorphous polymer such as polylactide (PLA)/poly vinylacetate (PVAc) blend [21,22], poly(vinylidene fluoride) (PVDF)/PVAc blend [23], and PVDF/polymethyl methacrylate (PMMA) blend [23]. These polymer blends are completely miscible at all compositions with a single, sharp glass transition temperature, while crystallization of PLA or PVDF is partially maintained and the degree of crystallinity, which controls the rubbery stiffness and the elasticity, can be tuned by the blend ratios. Tg of the blends are the critical temperatures for triggering shape recovery, while the crystalline phase of the semicrystalline PLA and PVDF serves well as a physical cross-linking site for elastic deformation above Tg, while still below T ,. 

Formation by chemical modification of hydrophobic polymers and subsequent physical or chemical cross-linking, for example, PVAc to PVA. 

Different types of water-based emulsions are used in EPI adhesives. The most common are poly(vinyl acetate) (PVAc) emulsion, ethylene vinyl acetate (EVAc) emulsion, vinyl acetate-acrylate copolymerized (VAAC) emulsion, acrylic-styrene (AcSt) emulsion or styrene-butadiene rubber (SBR) latex or modified versions of these emulsion types [1, 8, 9], It has also been reported that tri- or ter-polymer emulsions like vinyl acetate-butyl acrylate-hydroxypropyl methacrylate or emulsions with different combinations of block copolymers can be used [4], Emulsion polymers containing cross-linking functional groups are especially well suited [4,6, 9]. The choice of emulsion(s) will, to a large extent, influence the adhesive properties such as setting time, bond quality, heat resistance, and moisture resistance. EPI adhesive systems are, however, very complex and the total composition (including the choice of cross-linker) and the interaction between the different components will determine the properties of the adhesive. Due to this it is difficult to describe in detail the effect of choosing one type of emulsion over the other. 

The PVAc formulations currently available on the adhesives market are rather complex systems comprising a poly(vinyl acetate) dispersion, a film forming promoter, a cross-linking agent and/or a hardener. The use of modified poly(vinyl acetate) dispersions generally requires a thermal treatment to obtain the best results and the use of a specific cross-linking comonomer, such as N-methylolacrylamide (NMA), may cause a lower shelf-life of adhesives as well as formaldehyde emissions. 

PVAc-based commercial wood adhesives are evaluated using standard tests for non-structural applications, as reported in EN 205 [8], and they are classified in agreement with the standard EN-204 [9]. This standard allows to classify wood adhesives in 4 categories from D1 to D4. D1 adhesives show a good resistance only in dry conditions D2 adhesives should withstand a rather low water presence, such as in occasional exposure in kitchens and bathrooms D3 adhesives are suitable to come in contact with cold water, such as for outside windows and doors, kitchen and bathrooms furniture D4 adhesives are suitable to be used in extreme conditions (resistance to hot water). Vinyl acetate homopolymer can be used to formulate D1 or D2 adhesives. Vinyl acetate based adhesives cross-Unked with hardeners and urea-formaldehyde (UF) adhesives belong to class D3. Only the phenol-formaldehyde (PE), resorcinol-formaldehyde (RF) and melamine-formaldehyde (MF) adhesives, some special 2-component polyurethanes (PUs), and cross-linking vinyl adhesives belong to class D4. 

A wide range of adhesive types and chemistries are used to bond wood elements to one another (Table 2), but relatively few adhesive types are utilized to form the composites themselves. The vast majority of pressed-wood products use synthetic thermosetting adhesives. In North America the most important wood adhesives are the amino resins (qv), eg, urea-formaldehyde (UF) and melamine-formaldehyde (MF), which account for 60% by volume of adhesives used in wood composite products, followed by the phenolic resins (qv) eg, phenol-formaldehyde (PF) and resorcinol-formaldehyde (RF), which account for 32% of wood composite adhesives (12,13). The remaining 9% consists of cross-linked vinyl (X-PVAc) compounds, thermoplastic poly(vinyl acetates) (PVA), soy-modified casein, and polymeric diphenylmethylene diisocyanate (pMDI). Some products may use various combinations of these adhesives to balance cost with performance. 

FIGURE 2.2 Monomers can form different shapes of polymer, (a) Randomly coiled linear thermoplastics, e.g. PMMA. (b) Shghdy branched thermoplastics, e.g. PVAC. (c) Highly branched thermoplastics, e.g. polyurethane foam pre-polymer, (d) Cross-hnked polymers with trifunctional junctions, perhaps formed by reaction of (c), e.g. epoxy resin, (e) Cross-linked polymer with tetra-functional junctions, e.g. polyester casting resin. Source Brydson (1982). 

Entangled PVAc-spherical-HA 5 vol%, 5 Hz (Kalfus et aL [67]) Cross-linked PEA-silica 7 vol%, 1 Hz (Lequeux et aL [74]) - —Cross-linked PEA-silica... 

Figure 6.9. Temperature dependence of the LAM/HAM (low amplitude modulus/high amplitude modulus) for cross-linked polydimethyl siloxane (PMDS)-silica uanocomposite [vi= 0.15) and entangled PVAc-HA nanocomposite (uy= 0.05)...

FIGURE 1.19 SEM micrograph of (a) porous PTFE thin film. (Reproduced with permission from Lin, H.L. et al., J. Membr. Sci., 237,1,2004.) (b) Electrospun PVAc nanofiber film cross-linked with glutaraldehyde. (Reproduced with permission from Lin, H.L. et al., J. Membr. ScL, 365, 114, 2010.)... 

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