Page 156 - Handbook of Plastics Technologies

THERMOSETS

3.26 CHAPTER 3

TABLE 3.23 Sheet Molding Compound
Formulation
Polyester 26.2%
Low-profile resin 4.6%
t-butyl perbenzoate 0.3%

CaCO 3 6.2%
Zn stearate 0.9%
MgO 1.8%
Glass fiber 60.0%

resin and then wound onto a mandrel to form the shape of the desired product. The angle
of winding can be calculated to maximize properties. It is then oven cured, and the man-
drel is removed. This is obviously limited to hollow products of fairly simple shape, but
mechanical properties can rival metals. Typical uses are tanks, pipes, boat hulls, and mod-
ular housing.
Market importance of different processes. The major processes for unsaturated
polyesters may be ranked in descending order as follows:

• Sprayup
• Sheet molding compound
• Continuous laminating
• Filament winding
• Hand layup
• Bulk molding compound
• Pultrusion
• Injection molding
3.1.3.1.4 Properties. Properties of cured reinforced polyesters result from the com-
bined effects of (1) the process technique and (2) the type of formulation used in each pro-
cess (Table 3.24). Cast polyester lacks the benefits of fibrous reinforcement. Sprayup is
easy but uses short fiber and achieves limited compaction. Bulk molding premix uses short
fiber and high filler loading. Sheet molding compound uses longer fiber and less filler. Pre-
form uses fairly long fiber and does not suffer shear degradation during molding. Hand
layup can benefit from use of woven fabric, which contributes to higher reinforcement.
Pultrusion achieves very high reinforcement in the machine direction. And filament wind-
ing packs the maximum concentration of reinforcing fiber, and orients it to maximize its
reinforcing effect.

3.1.3.2 Vinyl Esters. The so-called “vinyl ester resins” are analogous to polyesters.
They are more expensive but more resistant to hydrolysis, so they are popular for chemi-
cal-resistant equipment (e.g., tanks, pipes, ducts, scrubbers, and towers) because they are
less expensive and more durable than stainless steel. Some other advantages are faster
cure, higher elongation and impact strength, adhesion to glass fiber reinforcement, and re-
sistance to heat aging.

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