How to Formulate PVC Plastisols?

Low cost 

- The primary reasons for the commercial success of plastisols are their low cost and ease of application. Many synthetic adhesives have been developed to improve the properties of PVC plastisol adhesives. However, no adhesive can match their low selling price.

 

Temperatures - Plastisols require temperatures on the order of 130°C to 400°C to "cure". However, they are commercially supplied as one component pastes without the need for metering or mixing prior to application. The service temperature range for most PVC plastisols is 0°C to 125°C.

 

Bond strength & Flexibility - Moderately high bond strength can be achieved to many common substrates. Long-term flexibility is a distinct advantage where the plastisol can accommodate relative motion between substrates and act as a vibration damper. This feature enables PVC plastisols to compete in both the adhesive and sealant markets.

 

Material Properties - The plastisol material can be formulated to be either a soft foam or a dense hard solid, which retains toughness even at low temperatures. Plastisols can be formulated with hardness ranging from 30 to 90 Shore A and tensile strength ranging from 750 to 3000 psi. It can be formulated to resist chemical attacks and provide good weatherability. It is self-extinguishing due to the chlorine groups.

A plastisol is a 100% non-volatile paste-like composition (sometimes referred to as "vinyl paste"). It consists of a physical mixture of finely sized PVC polymer particles, liquid plasticizers, and other additives. Common ingredients used in formulating PVC plastisols include:

 

PVC resin as base polymer

Polyvinyl chloride (PVC) resins are arguably the oldest and one of the most versatile thermoplastics. A wide variety of PVC resin types are available in the market today. They can be modified with a multitude of compounding ingredients to achieve a broad spectrum of applications. PVC resins are widely used in fabricated goods, including extruded, calendered and molded products.

PVC resins, in general, have limited solubility, compatibility and thermal stability. This inefficiency restricts adhesive usage to specific areas. However, PVC resin-based adhesives have found use in several forms including:
 

• solvent borne,
• waterborne,
• dry powders, and
• one-part non-volatile plastisols

The PVC resins used in adhesives are suspension or emulsion polymerized products. These polymerization processes are summarized in Table 1.
 

Key requirements of PVC resin
 

• Particle size: The particle size of the PVC resin must be small enough to permit it to stay in suspension without settling. This usually mandates a dispersion grade of PVC powder (about 1 micron in diameter).
• Solvation resistance: The resin must resist solvation at room or storage temperatures. This will help the final product to have a practical shelf life.
   

The PVC resins that are most often used in adhesives and sealants are copolymers and homopolymers of polyvinyl chloride copolymers. In some cases, vinyl acetate copolymers are also used.

 

Plasticizers

Liquid plasticizers have relatively good viscosity stability at ambient temperatures. However, when exposed to elevated temperatures in the 130°C to 400°C range, the plasticizer solvates the suspended PVC polymer particles. This results in a permanently fused product. This conversion from a liquid to a solid takes place in two steps as mentioned below.

 

STEP 1: Gelation

Gelation occurs at about 65°C and converts the plastisol to a semi-solid but weak structure, which will not flow. As the temperature is raised further, the polymer molecules increase their motion and become less entangled and compact. The figure shows, PVC particles (gray) dispersed in plasticizer matrix (pink).

 

STEP 2: Fusion
 

As they spread out, a phase inversion occurs. The solid resin particles originally dispersed in plasticizer convert to plasticizer (pink) dispersed in the resin (gray). Further heating to the maximum application temperature results in fusion, homogeneity, and the development of optimum strength. The exact temperatures required for these processes will depend on the formulation.


Some common plasticizers used in PVC plastisols include adipates, benzoates, and phthalates.

 

Fillers

Fillers such as calcium carbonate are used in formulating PVC platisols serve multiple critical functions:
 

• Increases paste viscosity
• Provides sag resistance
• Provides chemical stabilization
• Reduces cost
• Improves plastisol stability
• Helps maintain consistent material properties
   

Plastisol adhesive formulations usually contain only 20-25 percent by weight of PVC resin. The PVC resin component may be totally dispersion grade PVC resin or a blend of 75-85% dispersion polymer and 15-25% suspension grade blending resin. 

The plasticizer is as important to the formulation as the PVC resin. The plasticizer will determine both the rheological characteristics of the plastisol in the paste form as well as the physical properties of the fused plastisol once it is cured. These properties will depend on the concentration and type of plasticizer used in the formulation.

Other additives used in formulating PVC plastisols include heat stabilizers, acid scavengers, adhesion promoters, and crosslinking agents. A typical composition of a PVC plastisol is shown in Table 2.
 

The formulating of plastisols for adhesives and sealants presents several challenges. The main problem is that the liquid plasticizer will affect the:
 

1. Fluidity of the uncured resin
2. Final mechanical properties of the fully cured resin

Thus, making a hard, rigid cured plastisol is difficult if one also requires a very fluid uncured paste. There are several solutions to this problem allowing the formulator to develop products showing properties ranging from soft, rubber-like characteristics to a tough, hard solid. Here are some solutions to resolve these problems:

 

Resin blends
 

• Blends of small particle size PVC powder with larger PVC powders will provide a certain degree of latitude in providing the required viscosity.
• Copolymers of PVC containing low concentrations of vinyl acetate are also used in developing plastisols that cure at relatively low temperatures. These systems are especially useful on heat sensitive substrates or in production facilities that do not have high temperature baking ovens.

Plasticizer selection
 

The choice of plasticizer is perhaps the most significant tool in the formulators' arsenal. 
 

• Phthalates are commonly used as plasticizers in PVC plastisols. Plasticizers that are capable of homopolymerizing such as diallyl phthalate or epoxidized oils will provide a low paste viscosity but do not act as a strong plasticizer after fusion.
• New plasticizers such as the benzoate esters will also offer the formulator improved performance.² These have high solvating power to PVC. Thus, they can be used in relatively low concentrations without sacrificing paste fluidity.
• Some of the most successful plasticizers are blends such as dipropylene glycol dibenzoate (DPGDB) and triethylene glycol dibenzoate.

Organosol technology

Another answer to the problem of paste fluidity and a rigid cure is the use of what is called an "organosol". The organosol is like a plastisol except volatile organic liquids are added as thinners. The solvent acts as a viscosity reducer before fusion, but flashes off and leaves behind a more rigid product after processing. Of course in this type of formulation, the benefits of a non-volatile system are lost. Find out how to reduce VOC of adhesives & sealants.

 

Incorporation of fillers
 

• In certain applications, an increase in paste viscosity is needed. Adhesives and sealants that must be applied to vertical surfaces for example should have sag resistance or a thixotropic character. Considerable time may elapse between the time the adhesive is applied and the time the part is cured. Fillers, such as precipitated calcium carbonate, are commonly used to achieve these non-sag properties.
• Calcium carbonate provides a secondary benefit in its ability to remove HCL formed by any partial decomposition of the plastisol.
• Functional precipitated calcium carbonate is generally used in combination with ground calcium carbonate.
• Plastisols that do not run off the coated article when applied and heated are sometimes referred to as "plastigels". Inorganic fillers are also sometimes included in plastisols to even further reduce cost.

Heat stabilizers
 

Other ingredients commonly found in plastisol adhesive formulations are heat stabilizers whose primary purpose is to scavenge acid that may be released during cure. Conventional liquid epoxy resins often serve this function. The epoxy serves as a secondary plasticizer, acts as a stabilizer and helps to promote physical properties by crosslinking during cure.

 

Some common applications of PVC plastisols include:

 

Automotive industry

Their main application was to bond sheet steel to inner stiffener panels and to seal around the crimped panel edges. These adhesives are formulated as high solids and thixotropic pastes. They are applied generally as discrete dots or droplets (sometimes referred to as "Hershey drops").

In addition to structural fastening, these adhesives provide a degree of sound deadening and vibration/flutter reduction that is important in consumer products. Specific applications include automotive rear decks, door panels, and large appliance assemblies.
 

Most plastisol adhesives are soft and flexible after cure. They give about 50-100% elongation at room temperature and tensile shear strength on the order of about 400-700 psi on steel. Higher bond strengths are generally not needed in these applications because of the large bonded areas. Often the plastisol adhesive is used to eliminate or replace a certain amount of spot welds.

Plastisol adhesives normally have excellent adhesion to oily metals. They have, therefore, found an early niche market in the automotive industry. However, PVC plastisols degrade rapidly at temperatures greater than 200°C with the release of HCl. If released during cure, the acid will result in the blackening of the substrate.

 

Textile and carpet industry
 

Plastisol adhesives are also heavily employed in the textile and carpet industries. Textile bonding end uses include film lamination and tie coats. These applications most often involve film-to-fabric bonding. The predominant fabrics are nylon and polyester. 

Plastisol adhesives used for laminating textile products generally have a low level of inorganic filler and a relatively high level of PVC resin. Table 3 shows several PVC plastisol formulations that are used for laminating vinyl film to polyester textiles.
 

PVC plastisols are poised for significant evolution in the coming years. Manufacturers are increasingly developing eco-friendly formulations with reduced volatile organic compounds (VOCs). They are also exploring bio-based plasticizers to address environmental regulations.

The automotive, medical, and construction industries are expected to be key drivers of innovation. Plastisols are being engineered for improved performance, lighter weight, and enhanced durability. These options make the fate of PVC plastisols even more versatile.