Biobased Components in Hot Melt Adhesive Formulation

1 Biobased Components in Hot Melt Adhesive Formulation SpecialChem | Edward M Petrie - Jul 11, 2012. Introduction There seems to be a staggering amount of development activity related to Biobased polymers, but they have not, as yet, made a significant commercial impact in the Adhesive market. One reason for this is the lower volume of polymers used in adhesives when compared to the plastics used in the packaging, building, and consumer products sectors. Even so, the Adhesive industry is taking reasonable, controlled steps to develop Adhesive systems based on renewable sources, and nowhere is this more apparent than in the hot melt area. Hot melt adhesives are on the front lines for several reasons. One factor is the wide variety of large commercial applications especially in the environmentally sensitive areas of packaging. Another factor is that hot melt adhesives have grown and replaced other Adhesive types primarily due to favorable environmental factors.

2 The renewable value proposition will only add to this preference (Figure 1). As a result, one should look to hot melt adhesives in the near term for major application of Biobased raw materials. Figure 1: Current and future environmental benefits of Biobased hot melt adhesives This article will review the state-of-the-art regarding the Formulation and development of Biobased hot melt adhesives. It also analyzes the several driving forces for such development and the barriers that are likely to inhibit quick and large scale commercialization. Both primers and adhesion promoters are strongly adsorbed onto the surface of the substrate. The adsorption may be so strong that it has the nature of a chemical bond. Such adsorption is referred to as chemisorption to distinguish it from reversible physical adsorption. Organosilane adhesion promoters or primers are liquid products that form a very thin (usually monomolecular) layer between the substrate and the Adhesive .

3 Different chemical bonds are formed (1) between the adhesion promoter and the Adhesive and (2) between the adhesion promoter and the substrate surface. These bonds are often stronger than the internal chemical bonds within the Adhesive . These new bonds also provide an interface region that is more resistant to chemical attack from the environment. This article is intended to be a guide to the selection of an organosilane adhesion promoter or primer. The organosilane adhesion promoters vary by their inorganic and organic reactive groups, molecular stability, moisture sensitivity, solvent solubility, and cost. They are generally chosen by first matching the organic functionality to the base polymer to optimize bonding. However, the choice of the correct organosilane family or a type within a given family is often not a straight- forward task. Value Propositions of Biobased Hot Melt Adhesives Biopolymers are man-made polymers which are based on renewable raw materials such as agricultural feedstock.

4 Such feedstock include those that are well known and used in everyday practice ( , corn, starch, soy) and those that are extracted from biomass and further refined into polymers. This latter group includes the newer polymers that are refined to mimic today's common polymers ( , polyols and polyesters). It is extremely unlikely that biopolymers will completely displace petrochemical based raw materials in all Adhesive formulations. However, a more plausible scenario foresees renewable raw materials supplementing the petrochemical resources. Even an Adhesive Formulation that is only partly based on renewable feedstock can be an environmentally useful approach especially if the value proposition is not degraded by high cost or inferior properties. For a more complete understanding of the value propositions related to replacing petroleum based Adhesive products with those based on renewable resources, the reader is directed to a previous SpecialChem article 1 that attempts to determine if today's biopolymers represent a fad or if they are the preview to the future.

5 Hot Melt Adhesive Formulation Hot melt adhesives are usually composed of three primary Components : based polymer, tackifier, and wax as shown in Table 1, all of which are primarily been derived from petroleum. In addition, small amounts of various additives ( , antioxidants, UV inhibitors, plasticizers) are used for controlling the application and / or service properties of the Adhesive . Component Function Properties Base polymer Strength Strong Hot tack High molecular weight Viscous loss (peel (>10,000). strength) Tg usually less than 23 C. Physically crosslinks on cooling Tackifier Lowers viscosity Amorphous Improves wetting Low molecular weight Optimizes peel strength (<5,000). at a service temperature Tg usually greater than (via Tg) 23 C. Decreases the degree of chain entanglement Wax Increases setting speed Crystalline Lowers melt viscosity Low molecular weight (<2,000). Tg less than 23 C. Table 1: Components of a Typical Hot Melt Adhesive Thermoplastic polymers used in conventional hot melt Adhesive formulations generally fall into several major groups: vinyl copolymers such as ethylene vinyl acetate (EVA), polyolefins such as certain polyethylenes, block copolymers such as styrene block copolymers (SBCs), and thermoplastic polyurethanes.

6 The most common conventional tackifiers are derivatives of tall oil resin or hydrocarbon resins. The most commonly used waxes are synthetic polyolefin waxes. The move to raw materials based on renewable resources is illustrated in Figure 2 and described in the sections below. All of these renewable Components are commercially available today. However, their relatively small commercial use in the Adhesive industry has been due to high introductory prices, lack of suppliers, and difficulties in reformulating ( , not necessarily a "drop-in" alternative). Some Components such as tackifiers and waxes are easier to replace with renewable raw materials. Therefore you see several commercial hot melt adhesives that are not totally based on renewable sources. The goal of a 100% renewable product is mainly inhibited by the base polymer component. Figure 2: Trend to renewable raw materials as Components in hot melt adhesives. 2. In order to be a replacement for petroleum derived products, hot melt Biobased adhesives must have low price and have properties that are similar or better than the incumbent product.

7 In certain industries this can be quite difficult. According to a specification developed by The Procter &. Gamble Company for a sustainable hot melt packaging adhesive3 , the ideal Adhesive should exhibit the following properties: 100% derived from renewable resources Thermoplastic elastic behavior Tg of less than -10 C. Predominant amorphous character (low or no crystallinity). High elongation on break (at least greater than 200%). Relatively low melt viscosity (melt flow index greater than 10 dg/min). Thermal stability in the melt: after 72 hrs in air at 150 C (or a temperature 20 C higher than its softening point if greater than 130 C) the Adhesive must show variation in melt viscosity less than +/- 35%. Adhesive must meet toxicological, safety, and regulatory requirements suitable for the application Price should not be higher than about $5 USD / kg Available production of at least 5000-10,000 tons / year in 3-5 years. Waxes and Tackifiers The wax and tackifier Components have made the greatest strides in Biobased hot melt adhesives.

8 This is primarily due to the fact that Biobased wax and tackifier Components have been available for many years and their characteristics are well known to hot melt Adhesive formulators. Standard tackifiers for hot melt Adhesive and sealant formulations are either based on natural or petroleum based products. Synthetic, petroleum derived tackifiers are either aromatic or aliphatic resins. Examples of natural product tackifiers are rosin acid derivatives and other esters. Another natural product is that based on a class of materials known as the terpenes. These tackifiers are extracted from pine trees and the paper production processes and then converted into resins with the help of distillation and further processing. Other, more modern Biobased tackifiers are based on citrus resources. An example of this type is d-Limonene that is derived from the citrus monomer d- limonene. These natural tackifiers are summarized in Table 2. Companies such as Arizona Chemical, Pinova, and DRT specialize in producing natural tackifiers for adhesives and sealants.

9 Plasticizer Type Compatible Polymer Properties Pine rosin Provides excellent adhesion to a wide range Ethylene vinyl acetate of substrates due to Acrylics polarity Polyurethane Low, narrow molecular Styrene butadiene weight distribution copolymers (SIS and Low viscosity SBS). Available in a range of softening points Terpenes Provides pressure Polyolefins sensitive Adhesive Midblock of SIS characteristics copolymers Food contact approval Styrenated and terpene Light colored phenolic types are compatible with EVA, Low odor acrylic, and SBCs Terpene phenolics have excellent hot tack Citrus (d-Limonene based). Block copolymers Provide excellent pressure sensitive Butyl characteristics Polyisoprene Used in can sealants Table 2: Characteristics of Conventional Biobased Tackifiers for Hot Melt Adhesives In addition to the characteristics listed in Table 2, the tackifying agent must be compatible with the base polymer, and this is one of the problems in formulating a completely Biobased hot melt Adhesive .

10 Biodegradable tackifiers are not easily compatible with renewable polymers. Waxes are often used in hot melt formulations to lower surface tension and decrease melt viscosity. Conventional hot melt adhesives use petroleum derived waxes such as paraffin and microcrystalline wax. Certain waxes such as microcrystalline waxes also reinforce the hot melt by forming crystallites that resist deformation under load. These are used in formulations that require a relatively high degree of creep strength. Natural renewable waxes of various qualities are found as coatings on leaves, stems, berries and grasses that grow wild in many countries. The largest amounts of wax are produced by plants in locations where this coating is provided by nature to reduce the evaporation of moisture. Recently, due to the limited supply and increased price of paraffin wax, attention has turned to the use of natural waxes. Of these, the lowest cost waxes are based on highly hydrogenated triglycerides.