Biodegradable polymers based on renewable resources are being used as low-cost alternatives to petroleum-based raw materials.
Soy adhesives can be used for making semi-water-resistant plywood and for coating some types of paper.
Significant advances have
been made over the past 20 years in the development of biodegradable polymers.
These materials have been developed in a variety of forms, and thus have
potential uses in a range of industries. Many of these polymers are suited for
adhesive applications, such as environmentally friendly packaging, recyclable
envelope adhesives and biomedical devices.
Biodegradable polymers based on renewable resources are also being seen as
low-cost alternatives to petroleum-based raw materials. Industries that develop
these materials will continue to see growth, as the price of crude oil keeps
climbing and the availability of fossil fuels begins to dwindle. Applications
for such bio-based materials are widespread in the areas of resins, coatings
The development of biodegradable adhesives goes hand-in-hand with the
development of biodegradable plastic products. In order for the finished
product to be completely biodegradable, all of its components must be
biodegradable. A range of potential biodegradable products is available,
including the following.
- Packaging materials (trash bags, loose fill foam, food
- Consumer goods (egg cartons, razor handles, toys, straws,
- Industrial products (planter boxes, compositing bags, fishing nets,
- Medical applications (drug-delivery systems, sutures, bandages,
- Coatings (barrier coatings with low moisture vapor transmission of
paper and plastic film)
- Hygiene products (flushable sanitary products, diaper backing
Many of the polymers developed specifically for these products may also be
suitable base polymers for biodegradable adhesive systems.
This article identifies the key issues for biodegradable adhesives, including
- The market potential and growth opportunities for biodegradable
- The various types and composition of biodegradable polymers that may
be appropriate for adhesive formulations
- Recent commercial activity in this area
Definition of "Biodegradable"
The failure of early
biodegradable polymers to properly degrade has led the ASTM to create a definition
for what constitutes “biodegradability.” Biodegradability means that a product
is “capable of undergoing decomposition into carbon dioxide, methane, water,
inorganic compounds, or biomass in which the predominant mechanism is the
enzymatic action of microorganisms that can be measured by standardized tests,
in a specific period of time, reflecting available disposal conditions”.1
Many so-called biodegradable polymers are actually bioerodable, hydro-biodegradable
Hydro-biodegradable and photo-biodegradable products are degraded or broken
down in a two-step process: 1.) Hydrolysis or photo-degradation; and 2.)
Further biodegradation as defined above by the ASTM. Products can also break
down by a single-phase process (water soluble or photo-degradation) where the
residue is not further degraded by organisms. Bioerodable polymers are capable
of degrading without the action of microorganisms, at least initially. Their
degradation processes may include dissolution in water, oxidative embrittlement
or UV embrittlement.
All of these polymers come under the broader category of “environmentally
degradable” polymers. For the purpose of this article, the term “biodegradable”
shall also imply “environmentally degradable.”
Most synthetic polymers are not biodegradable. Polymers such as polyethylene
and polypropylene can exist in the environment for many years after their
disposal. Biodegradable polymers are generally obtained by way of
polymerization of bio-based raw materials. These raw materials are either
isolated from plants and animals or synthesized through modern industrial
processes. Examples of biodegradable polymers are provided in Table 1.
Market Potential of Biodegradable Polymers
The global biodegradable
polymers market is currently estimated at 114 million lbs.2
Average annual growth rates are far in excess of the GDP, with forecasts for
the market to be well over 200 million lbs by the end of the decade (see Table
Packaging, which includes loose-fill packaging, made up nearly 47% of the total
biodegradable polymers market in 2005. However, compost packaging will
represent nearly 50% of the market by 2010. Other important products, such as
medical/hygiene, agricultural, and paper coatings, accounted for 11% of the
total applications in 2005.
The North American biodegradable polymer market has not progressed as rapidly
as it has in Europe and Japan.
The major drivers for the U.S.
market are mandated by legislation and prospective increases in landfill cost.
For significant growth in North America,
systems or infrastructures must first be installed to collect and process
biodegradable polymers, consumers must be willing to accept the inconvenience
and cost, and biodegradable products must be viewed as a realistic and
available alternative to waste disposal by all parties.
Starch-Based Biodegradable Polymers
Starch is a pure, natural
biopolymer found in the roots, seeds and stems of plants such as corn, wheat,
and potatoes. It is suitable for chemical modification into a thermoplastic
material for various applications. Starch is fully biodegradable and based on
renewable materials. Thus, its use in commercial adhesive compounds and in
plastic materials will minimize environmental damage.
Starch degrades by molecular breakdown resulting from enzymatic attack on the
glucosidic linkages between sugar groups. These products have starch contents
that vary significantly. The starch content needs to exceed 60% for significant
material breakdown, although most starch-based biodegradable polymers have
starch contents of 10-90+%. As the starch content is increased, the polymers
become more biodegradable. At low starch contents, the starch particles act as
weak links in the polymer matrix and provide sites for biological attack.
Starch could be the original biodegradable adhesive. It plays a very large part
in industrial production, especially the packaging industry. Starch-based
adhesives are principally used for bonding paper products and other porous
substrates. Most corrugated boxboard for making cartons is easily bonded with
Starch-based adhesives, made from potato and other starches, are principally used for bonding paper products and other porous substrates.
There are many benefits to
starch adhesives. They are readily available, inexpensive and easy to apply
through water dispersion. They are considered to be the least-expensive class
of paper-packaging adhesive. Formulated starch adhesives can be applied hot or
cold. The adhesives are generally provided to the end user as powder and mixed
with water prior to use to form a relatively thick paste. Starch and dextrin
cure through the loss of moisture; since these adhesives cure to a
thermosetting structure, they have excellent heat resistance. Another advantage
is their very slow curing rate, allowing ample assembly time. Disadvantages
include poor moisture resistance and mold growth.
Although starch-based adhesives have been used for many decades, there are
several important reasons why these natural adhesives will not be entirely
replaced by synthetic products. The following advantages ensure that they continue
to fill particular niches in the marketplace.3
- Availability is good and cost is relatively low
- Quality is stable
- Good adhesion to cellulose and many porous substrates
- Insolubility in oils and fats
- Non-toxic and biodegradable
- Heat resistance
To meet the requirements of various modern applications, starch-based polymers
may be blended with higher-performance biodegradable polymers, such as
aliphatic polyesters and polyvinyl alcohols. Adding starch to a polymer mix reduces
the volume of synthetic materials required, thus reducing the overall material
cost. Varieties of biodegradable starch-based polymers include thermoplastic
starch, starch synthetic aliphatic polymer blends, starch-polybutylene
succinate (PBS)/polybutylene succinate adipate (PBSA) polyester blends and
starch-ethylene vinyl alcohol (EVOH) or polyvinyl alcohol (PVOH) blends. The
characteristics of these materials are summarized in Table 3.
Polyvinyl alcohol (PVOH) is
blended with starch to produce readily biodegradable polymers. The PVOH is very
water soluble, and the starch-PVOH blends are, therefore, degraded by way of
hydrolysis and biodegradation of the sugar molecules. Table 4 identifies some
of the starch-PVOH blends that are commercially available.
As starch is fully biodegradable and easily renewable, it will continue to be
an important component of the biopolymer industry.
Other Plant-Based Raw Materials
Several other biologically
based raw materials have been used in adhesive systems. Since the beginning of
time, plants have been known to act as feedstock for a variety of chemical
applications. As of late, there has been a return to agriculturally based
chemicals. The development in this area is primarily due to environmental regulations
and conservation, but biodegradation of waste has contributed as well. In
addition to starch, much work has been done on adhesives based on casein, a
material derived from milk.
Linseed oil is a common feedstock for resins and coatings. It can be used to
derive several high-performance polymer resins, including polyester amide. Hemp
oil-based products are developed for exterior coatings. The material boasts
high environmental awareness and is derived from a renewable resource.
Castor oil and soybean-based polyols are examples of plant materials that have
been successfully incorporated into polyurethane resins. Epoxidized oils are
synthesized by reacting vegetable oils (typically soybean and linseed) with
peracids or hydrogen peroxide. These epoxidized oils are used as biodegradable
plasticizers for adhesives and plastics. Long-chain fatty acid dimmers derived
from vegetable oils are reacted with a slight excess of primary amines to
synthesize polyamides, which are commonly used as curing agents in epoxy
coatings and adhesives.
potentially hydrolysable ester bonds of polyesters make them significant raw
materials in the development of biodegradable polymers. The polyester family is
made of two major groups: aliphatic (linear) polyester and aromatic (ring
structure) polyesters. Biodegradable polyesters that have been developed
commercially and that are in the development stages are shown in Table 5.
All polyesters degrade
eventually, with hydrolysis being the dominant mechanism. Aliphatic polyesters
are commonly used because they are more readily biodegradable than aromatic
polyesters. Synthetic aliphatic polyesters are easily biodegradable in soil.
These aliphatic polyesters are, however, more expensive and lack mechanical
strength when compared to conventional plastics. Aliphatic polyesters are
frequently combined with starch to reduce material cost. Compositions are
blended with nanoclay reinforcement to improve mechanical properties and
barrier performance in food-packaging applications. Polylactic acid (PLA) is
the most widely used biodegradable polyester.
Lactic acid is produced principally by way of microbial fermenting sugar
feedstock. Making PLA requires 30-50% less fossil fuel than polymers synthesized
from hydrocarbons. Variation in polymerization conditions permits the synthesis
of several grades of PLA. Polylactic acid degrades primarily by hydrolysis and
not through microbial attack. It does not biodegrade readily at temperatures
lower than 60°C due to its glass-transition temperature of close to 60°C.
PLA has high polarity, making it difficult to adhere. Tie layers generally must
be used to provide a bond to non-polar PE and PP in multi-layer structures. PLA
has excellent heat-sealing performance. PLA-based materials, such as those
produced by NatureWorks LLC under the trade name of NatureWorks, are most
commonly used in packaging as thermoformed products, such as drinking cups,
takeout food trays and other containers.
Polycaprolactone (PCL) polyesters are low-viscosity synthetic aliphatic
polyesters. Their relatively high cost can be overcome somewhat by the addition
of starch. Derived from non-renewable sources, PCL is fully biodegradable
through a single process of composting at 60°C. Without additives, it
completely biodegrades after about six weeks in compost activated with sludge.
Processing additives increase tensile strength but lower biodegradability. See
Table 6 for a list of companies that supply PCL materials.
Soybean flour is formulated
into adhesive systems as the main polymeric component, incorporated as
extenders for phenolic resins, or blended with casein or sodium-silicate
adhesives. Soybean glue is derived from protein. These adhesives are cheap and
can be used for making semi-water-resistant plywood, and for coating some types
of paper. They are primarily used to manufacture plywood, and set at room
One-package soybean glue formulations are dry powders. Soybean powder contains
both protein and carbohydrate. For adhesives, the flour is generally dispersed
in aqueous sodium hydroxide, and other alkaline bivalent metallic ions - such
as calcium hydroxide - are incorporated to lengthen working time and improve
Soybean glues are generally considered to have limited water resistance, but,
like casein glues, recover their strength on drying. They are susceptible to
mold growth, but a range of preservatives is available, such as
pentachlorophenol, copper naphthenate and tributyl tin oxide. Fungicides also
provide some degree of termite protection.
Many polyfunctional materials are used as crosslinking agents for soybean
proteins. Typical denaturants and crosslinking agents include sulfur compounds,
such as carbon disulfide; soluble metal salts; epoxies; and formaldehyde
donors, such as dimethylurea or trimethylphenol. Small proportions (usually
under 1% of the dry weight of soybean flour) are sufficient. Fillers, such as
wood flour, walnut-shell flour and clay, reduce cost, but they also lower the
adhesive’s performance properties.
Soybean glues, like blood glues, can be safely used in the cold for wood
lamination. However, for rapid production of assemblies, where several cure
cycles are performed per hour, it is necessary to use temperatures as high as
140°C with pressures of 175-200 psi applied to the joint.
Modern soybean adhesives are made by combining hydrolyzed soy protein with
phenol resorcinol formaldehyde. Compared to standard phenol resorcinol
formaldehyde adhesives, this hybrid adhesive has a faster cure time and could
bond wood with moisture content as high as 150%. Thus, soy-based adhesives can
join green lumber, which allows lumber mills to operate with less energy and
fewer harmful emissions. New research at Iowa State University has resulted in formulations
that can replace up to 70% of the phenol formaldehyde in adhesives used to bond
a variety of wood and fiber-based composite products.
Other Biodegradable PolymersWater Soluble Polymers
There are two primary water-soluble polymer types: polyvinyl alcohol (PVOH) and
ethylene vinyl alcohol (EVOH). PVOH is readily biodegradable and water soluble.
It is commonly used in many conventional adhesive systems. It is often blended
with polyvinyl acetate (PVAc) for wood adhesives. The degradation of PVOH is
influenced by its crystallinity and molecular weight. The main degradation
mechanism is dissolution in water. Biodegradation in soil is very slow.
EVOH is another water-soluble synthetic polymer, often used as an oxygen
barrier layer in multilayer film packaging. It can be used as an effective
tie-layer in laminating multilayer product. The high cost of EVOH is a
significant barrier to its widespread use in biodegradable plastics.
Photo-degradable polymers are polymers into which light-sensitive chemical
additives or copolymers have been incorporated in order to weaken the bonds of
the polymer in the presence of UV radiation. Photodegradable polymers are
designed to become weak and brittle when exposed to sunlight for prolonged
periods. Photosensitizers used included diketones, ferrocene derivatives and
carbonyl-containing species. These plastics degrade in a two-stage process,
with UV light initially breaking the molecular bonds.
Controlled Degradation Through Additive
Additives that impart controlled degradation behavior to conventional
thermoplastics, as well as to inherently biodegradable polymers, are becoming a
popular strategy due to price competition. Such additives are known as
prodegradant concentrates, and are generally based on catalytic transition
metal compounds, such as cobalt stearate or manganese stearate. The additive is
typically used at levels of 1-3% and, therefore, may be a practical solution
for many biodegradable adhesive formulators.
The principal company that has developed these prodegradant additives is EPI
Environmental Technologies, Conroe,
TX. Their products are trade
named TDPA, or Totally Degradable Plastic Additives. Polymers modified with
TDPA progressively degrade to lower and lower molecular weights. They become
brittle, disintegrate and are ultimately digested by microorganisms.
Recent Commercial Activity in Biodegradable Adhesives
There are only a few major
players in the global biodegradable polymer business, including NatureWorks LLC
in North America and Novamont and BASF in Europe.
Many Japanese companies are involved, but they have relatively small production
volumes, including some pilot plant operations. There are far less companies
active in developing biodegradable adhesives. Following is a summary of recent
activities in this area.
Bio-based raw materials are now finding their way into adhesives applications
where recycling and environmental concerns are important. Examples include a
family of harmless, reactive, sugar-based monomers called ECOMER (EcoSynthetix
Inc., Lansing, MI). Copolymerization of these monomers with
acrylic monomers has resulted in sugar-based acrylic pressure-sensitive
adhesives for recycling. This adhesive is being used in environmentally benign,
self-adhesive postage stamps.
PSAs are also being developed at the University
of Delaware from plant
oil derivatives, such as fatty acid methyl esters. The rheological properties
of the resulting polymers are comparable to petroleum-based polymers used in pressure-sensitive
Natural-based adhesives have also been formulated from components in the
forestry industry. A number of countries have made efforts to extract adhesive
for particleboard manufacture from the barks of their indigenous trees. These
woods generally have a high tannin content. This eliminates the need for
relatively expensive phenol resins. Special formulations have been prepared
with the combinations of tannins with melamine formaldehyde, phenol
formaldehyde and isocyanates. Adhesives using sulphite waste liquor from the
forest industry have also been developed as a particleboard adhesive.
The most common form of rosin adhesive is made from the oleoresin of the pine
tree. This material is used either in solvent solution or as a hot-melt mastic.
It has poor resistance to water and oxidation. Bond strengths are moderate and
develop rapidly; thus this adhesive is often used for temporary
One of the most tenacious bio-adhesives in existence is that produced by the
common sea mussel. The mussel depends on its ability to attach to solid
surfaces for its survival. Poor adhesion for a mussel means that it can be
dislodged from its marine home (generally wet rock, wood, or the steel hull of
ships) and crushed by waves or lost to some other nautical fate.
Chemists have long known that mussels secrete a material that consists of a
hardened matrix of proteins. These proteins form extremely tough fibers that
adhere to almost anything, and under almost all conditions. Exactly how these
proteins link together to provide an adhesive material, called byssus, has
remained a mystery until recently.
Researchers at the University
of California-Santa Barbara
have studied the mussel and concluded that the secret to the mussel glue’s
stickiness seems to be 3,4-dihydroxyphenylalamine (DOPA). DOPA is an amino acid
found abundantly in the mussel’s bonding proteins.4 Further research at Purdue University
indicates that another key ingredient in the mussel adhesive is iron.5,6
The mussel obtains iron by filtering it directly from its surrounding water. It
uses iron as a “crosslinker.” A single iron atom can bind to three DOPA side
chains. Other bio-available metal ions do not appear to bring about this
Unfortunately, the amount of adhesive protein that can be directly gained from
marine organisms, such as the mussel, is quite small and expensive to harvest.
Thus, we don’t believe that adhesive manufacturers will replace their reaction
vessels with mussel farms. However, the biosynthesis of these marine-adhesive
proteins can be copied, and production methods are possible and under
It is believed that adhesives having molecular structure similar to the mussel
secretions will be biodegradable and provide strong and durable structures. One
goal of trying to mimic mussel adhesives is the preparation of better marine
adhesives that can cure underwater, stick to almost any substrate and last a
lifetime. However, the value may be in another water-rich application - medical
adhesives. The ability of the material to bond to nearly any surface - not to
mention its biological origin - make it well suited for many internal medical
applications. Man-made mussel adhesives could be used for wound closure, nerve
reconstruction, or when one might need a scaffold upon which to grow cells and
build new tissue.8